If you're steamed by your mobile bill and shopping for a carrier, you might take a look at Google's Project Fi.

Although the service was invitation-only when Google launched it in April, 2015, the company has since opened it to everyone. The network covers the US, and earlier this week Fi teamed up with European network Three to bring service to 15 countries.

Cost and Features

Project Fi is cheap. Twenty bucks a month buys unlimited talk and text, plus Wi-Fi tethering. With data, you pay for what you use. Each gigabyte costs $10. Just $70 gets you voice, text and 5GB of data. The coolest part: Google credits any bandwidth you don't use. No, really. If you pay for 3 gigs and use just 2.2, $8 goes back to you. Also, contracts are month-to-month–no long-term commitment here.

Another neat feature? Project Fi automatically connects to available open Wi-Fi networks. Google maintains a database of "high-quality and reliable" networks, and switches to one whenever possible to save you data. Calls continue uninterrupted as you move from Wi-Fi to cellular or vice-versa. (Google uses Hangouts for this.)

What's the Catch?

Just three phones work with Project Fi at the moment: Google's Nexus 6P and Nexus 5X, and the older Nexus 6. The 5X and 6P are among the best Android phones right now, but still, you are locked into Google's handsets if you want to use Google's network.

Beyond the limited hardware, Google's network leaves a little something to be desired. Project Fi relies on Sprint, T-Mobile, and US Cellular for wireless networks, switching among them depending upon signal quality and your location. Trouble is, these guys are among the spottiest of the major carriers. According to a Root Metrics coverage map, T-Mobile and Sprint ranked lowest in coverage. US Cellular has even less coverage. It's important to look at the combined coverage of all three carriers, however, since Fi switches between them regularly. If you're unhappy with your coverage on of these carriers where you live, service from the other two carriers could make up the difference. Still, if coverage is generally not great in your area, think twice about embracing Fi. You may also want to check the coverage in any cities you regularly visit.

Even with these limitations, Fi is cheap enough to warrant a closer look. New Nexus phones are expected this fall, so if you're looking to upgrade your handset, you might want to change your carrier, too. One cool thing about Nexus phones: they're unlocked. So if Project Fi doesn't work out, you can take the phone to whatever carrier you want.