Launched in the US in April, Project Fi is Google's foray into the mobile wireless service industry. It offers subscribers the familiar ability to call, text and surf the Web on their Google Nexus 6 smartphones while letting them control how much they pay for data per month. It differs from most other mobile service providers by using Wi-Fi as the default communication method for voice and data, and it smoothly switches between US carriers Sprint or T-Mobile's cellular and data networks when a hotspot isn't available.

Rather than describing its service as a competitor that undermines existing carrier partners, Google considers Project Fi an experiment that will hopefully push other companies to provide cheaper and more efficient data plans.

We tested the service in the San Francisco area across several days to check out what it's like using the tech giant's budding "network of networks."

Enlarge Image Josh Miller/CNET

Editor's note: Because the user experience with wireless carriers can vary greatly depending upon a variety of factors -- especially location -- CNET does not rate wireless carriers. We can, however, tell you everything you need to know about the major US carriers. We invite your feedback and ratings in our user opinions section. The more input we get from around the US, the better our users can evaluate coverage at a national level. For more information, please see CNET's Quick Guide to Cell Phone Carriers. In addition, because this is currently a US-only service, all pricing information is given in US dollars.

The basics

Project Fi primarily defaults to local Wi-Fi networks (the same kind you set up at home, or connect to at a coffee shop or airport) to patch calls and browse the Internet. Because some Wi-Fi spots are public, Google promises to encrypt your data so it's protected (yes, even the password-protected networks that belong to those small businesses still pose a security risk). If Wi-Fi coverage is not available or too weak, the network has partnered with US carriers Sprint and T-Mobile to use their cellular network.

Enlarge Image Josh Miller/CNET

As the first two major networks in the US to bake Wi-Fi calling into their coverage, this makes sense. T-Mobile and Sprint launched Wi-Fi calling in 2007 and 2014, respectively, with the former adding seamless hand-over from Wi-Fi to voice-over LTE. (As a side note, AT&T also rolled out its Wi-Fi calling service for some devices in October 2015).

Switching smoothly between networks is crucial since users won't notice when they're being ported over to one or the other. They also won't have their calls dropped just because they are leaving one network, nor have to take any action to switch over to another network. In addition, when Project Fi automatically connects to the one that is the faster and more reliable network of the two.

The process

As of now, if you want to participate in the service, you'll need to do two things: request an invite using a Gmail address and have a Nexus 6 phone (the Americas variant, model number XT1103). That's because there's a specialized radio inside the phone that supports multiple cell networks, and it works with the unique Project Fi SIM card, which also has multiple-carrier support. If you don't have the device, you can purchase an unlocked model from Google or other third-party retailers. (We bought one at Amazon.)

Once you get an invite, you'll be linked to the sign-in site where you need to enter your ZIP code to make sure you have coverage in your area. You can transfer your current number or get a new number altogether. If you elect to receive a new number, Google will suggest local area codes to choose from based on your ZIP code. This number will live on a cloud as your Google Voice number as well, and you can make calls from other devices with it too, like a tablet or laptop.

Enlarge Image Lynn La/CNET

After that, choose your data plan. There is no annual plan and the bare-minimum monthly rate is $20 plus taxes and fees. That includes unlimited calls and texting in the US, unlimited international texting in the US and 120-plus countries, and Wi-Fi tethering. If you want data, that's $10 a month for every 1GB of data (so $20 for 2GB, $50 for 5GB, all the way to $100 a month for 10GB) -- all of that is on top of that base $20 per month.

You will be charged upfront for your data at the beginning of the month, but if you don't use all of your allotted data, that amount will be refunded to you. For example, if you selected a 4GB plan but only used 3.1GB by the end of the month, you have 900MB leftover and will receive $9 back. The same goes if you exceed your data plan too -- if you go 900MB over your plan, you'll be charged an additional $9 on next month's bill.

(T-Mobile offers a similar rollover service called "Data Stash," which lets you use any untapped data in your plan for up to 12 months. AT&T does too, but the data expires at the end of the next billing period.)

When you're done signing up, Google will ship you a Project Fi nano-SIM card in the mail.

The experience

Data speeds

I tested 4G LTE data speeds in our San Francisco office. Results from Ookla's speed test app varied widely across multiple days and times. Download rates ranged from 3 to 13Mbps, while upload rates range from 1 to as high as 18Mbps. Out of seven trials, the average was 5.48Mbps down and 9.17Mbps up.

Enlarge Image Lynn La/CNET

For real-world, everyday usage testing, speeds were slow for the most part, but tended to be faster in the evenings. Out of three trials, loading the CNET mobile and desktop site took 16 and 18 seconds respectively, and downloading and installing the 44.52MB game Temple Run 2 took 5 minutes and 3 seconds. Downloading the 1.7GB movie "Gravity" once took a whopping 2 hours and 4 minutes, but during another attempt, it took 25 minutes and 16 seconds.

There's no built-in way to tell if the handset is using Sprint or T-Mobile's network at any given time (though you can install a third-party app like SignalCheck Lite that will tell you). It's important to note that you should take these results with a grain of salt. Data speeds differ widely depending on several factors, such as location and time of day. What I've observed here is just a minuscule sample size from our San Francisco location.

Lastly, as Android Central pointed out, if your Nexus is using Sprint's network, you can't use data and voice simultaneously -- a known issue that has affected phones using CDMA technology before.

Making calls

Making calls was simple and easy and doesn't require any setup. I made calls to landlines and cell phones and call quality was reliable -- audio was continuous, there was no extraneous buzzing or static and I was able to hear the person on the other end of the line clearly. Likewise, my partners reported that I sounded clear and easy to understand as well.

For Wi-Fi calls, I made sure that I had no cellular network signal before dialing a number and that the feature was toggled on in Settings. Calls that were being patched through this method showed the name of the Wi-Fi network on the dialer. When consistent, audio was very clear, easy to understand and appropriately loud. There were instances, however, when audio cut in and out, but it didn't last for very long. All in all it remained relatively steady.

Google Project Fi (Wi-Fi calling) quality sample Your browser does not support the audio element.

The competition

Google isn't alone in its wireless endeavors, and if you're considering Wi-Fi only networks, there are alternatives.

Republic Wireless charges $10 per month for talk, text and Wi-Fi, and $15 per gigabyte of data. If we choose to go with a 3GB plan, it would cost $55 per month. Excluding taxes and fees, that's $5 per month more than the same plan on Project Fi. You will, however, be credited for any unused data, and have a wider and cheaper lineup of devices to use, which include the $99 Motorola Moto G (2013) , the $129 Moto E and the $299 Moto X .

Scratch Wireless offers a different structure. While Wi-Fi tethering and texting is free, users have to pay for voice and data usage on a pay-as-you-go system. Though there are different usage tiers that span as short as 24 hours and as small as 50MB, let's revisit the 3GB example earlier. On Scratch, unlimited voice for 30 days cost $15 and 1GB costs $25 for 30 days. That means it'll cost about $90 to have all the same services as Project Fi and Republic Wireless. Currently, the carrier is selling only the Coolpad Arise for $99, but you can purchase other Scratch-certified devices from retailers like eBay.

Both Republic and Scratch Wireless use Sprint's network when Wi-Fi is unavailable. For a deeper dive on Project Fi's implication on other wireless services, check out Maggie Reardon's take.

Enlarge Image Lynn La/CNET

The takeaway

I was disappointed by the slow 4G LTE data speeds I experienced on Google Project Fi. Even though my observations offer just a narrow sliver of the network's possible coverage, benchmark tests were consistently slow compared to speeds I've previously seen from Sprint and T-Mobile.

Another major drawback is the fact that it's only compatible with the Nexus 6 for now. The premium phone starts out at $500 unlocked (for the 32GB model). Though you can pay it off in installments, the hardware will be beyond the budget of many people.

While the service pricing for Project Fi looks good at first, note that it's most competitive for individuals. Many big carrier family plans will offer better deals for two or more people sharing data on the same plan.

However, I did find Project Fi's pricing structure, an aspect of other wireless networks that many find to be frustrating, to be extremely user friendly. Its transparency is reassuring, and the fact that you don't have to worry about overshooting or letting your monthly data allotment go to waste is a relief. And obviously, Project Fi's biggest draw is the low month-to-month cost (again, for individuals). Depending on your data usage, your bill can be incredibly low, and getting credit for data you don't use means more money back into your pocket.