I have a soft spot for Windows on phones. Something about the clean, simple interface appeals to me as an alternative to the relative clutter of Android and Apple's iPhone, and chats I've had with Windows phone fans do make me believe that there are people out there better served by this approach to mobile software.

That's why it pains me to see the Windows 10 update in the Lumia 950 fall so short of its promise. The hardware itself is fine, if disappointingly uninspired. But the software, especially the buggy new Continuum feature that's supposed to let you turn your phone into something approaching a full-on Windows PC, only highlights Microsoft's inability to meaningfully keep up with Android and the iPhone.

Yes, the phone has a gorgeous display that's easy to read outdoors and camera image quality that keeps me smiling, especially in low light. I can even deal with the plastic construction because of the Lumia 950's removable battery and microSD card slot, which many metal phones don't have. The Cortana voice assistant is also better than Apple's Siri in my book.

But when you consider all the things that make up a modern mobile operating system -- mobile payments, access to limitless apps, software extras that work consistently well -- Windows 10 doesn't fix enough past problems and only introduces others. I was hoping for more with Windows 10 on phones, wanting to believe in the promise that Continuum would give the Lumia a competitive edge by boosting your productivity. It doesn't.

At the end of the day, the Lumia 950 costs too much and offers too little for all but Windows diehards to take seriously. It's been five years since Microsoft rebooted its old, failing Windows Mobile software into the Windows phone platform we see today, and the company still hasn't been able to work out the kinks. Microsoft's future as a world class software-maker for mobile phones is looking dim.

Pricing and availability

The Lumia 950 starts at $550 in the US, but you'll need to check with carriers, since their prices and payment plans will vary. AT&T, for example, sells it for $600 outright, $150 with a two-year contract, and for monthly installments that vary between $20 and $30 per month depending on which of its three Next plans you choose. The larger, 5.7-inch Lumia 950 XL will cost more, and goes on sale before the end of 2015.

Continuum: The phone becomes the PC (or TV)

See phone contents on a larger display

Can use wired or wireless keyboard, and mouse

Dock or HDMI port connection options

What if your phone could double as a makeshift PC? That, in a nutshell, is the promise of Continuum. It's arguably the big distinguishing feature of Windows 10 phones and something that doesn't really exist in the Android or iPhone realm. It should work with most future phones with Microsoft's mobile operating system (the low-end Lumia 550 won't have it), but the Lumia 950 is the first model to boast the functionality.

How do you use the phone as a PC? Pair a Bluetooth keyboard and mouse to the Lumia, and use a PC monitor or TV as the primary screen. For any of us who have had to transition from writing a bevy of one-line emails and texts to writing some 1,000-plus word documents from a hotel room, the idea is instantly enticing. And that's why I tested it by using it to write this phone review. (In fact, I'm typing this on my phone -- by way of a paired keyboard, monitor and mouse -- right now.)

The catch is that you need to tether to your big-screen display, through two possible methods, and they're both something of a pain. If the TV has built-in support for the wireless Miracast video standard -- or if you have a Miracast dongle plugged into the TV -- you can beam the signal from the phone through the preloaded Continuum app. Alternately, you can go with a more reliable wired setup, using the Microsoft Display Dock (which is what I used) -- it costs $80 in the US and AU$149 in Australia. (You'll also need the right cables.)

James Martin/CNET

Once you get it working -- in a living room, a hotel room or conference room -- you can share stuff like a PowerPoint presentation or home video with a group of people on the big screen. Then undock the phone, and resume working on the exact same project seconds later while walking down the street.

The idea is really cool, and once I got going (around this paragraph, actually) things started to really flow. There were a lot of false starts, however, and some major setup hurdles that make me seriously question Continuum's reliability. First, you need all the right hardware and cables, including an HDMI and USB Type-C cords. A wired or wireless keyboard and mouse are optional, too, but you can also navigate from the phone screen (while also using the phone independently).

Continuum setup and use

I had some issues with connection and pairing, and the third-party HDMI dongle that Microsoft sent as an alternative to its homegrown Display Dock puck didn't even work when I went to activate it -- its website said it didn't yet support Windows 10. Luckily, I had the Display Dock as a backup. Microsoft's Continuum app on the phone gives you a partial walk-through, but you'll still have to work out some things on your own. As promised, the image scaled well on the computer monitor I used, but didn't quite fit the frame of the 50-inch Samsung TV panel. At least only a sliver of image was cut off.

Navigating around the phone contents on a larger screen worked pretty well. Until it didn't. The phone spontaneously rebooted several times during my testing, but only while using Continuum, so that seems like the cause. One major use case is to craft documents using Microsoft Office. Keyboard shortcuts don't work the same, which means you'll do most of your formatting using the toolbar or mouse submenu. Not being able to paste or undo with keyboard shortcuts hampered my workflow.

James Martin/CNET

The cursor doesn't quite keep up when you're typing quickly either, so there's a bit of visual turbulence as your fingers fly, and without automated spell check in the Word doc, it's hard to correct errors -- sometimes you will see suggestions appear on the phone screen, all the way down by your elbow.

I do like the freedom of seeing what I'm doing on a larger, easier-to-read screen, but I'd much rather use a laptop for projects. Still, the option to use the HDMI dongle on your TV is the closest you can come to Google Chromecast for Windows phones (Chromecast does work with Windows laptops, though).

More Windows 10 software: Iris scanning, cleaner look

Windows Hello

Interface refresh

Customization tweaks

Besides Continuum, Windows 10 brings some nice additions to the Lumia 950. While every other phone is embracing the fingerprint reader in various locations, Microsoft carried over Windows Hello, its iris-scanning biorecognition software, from the Surface tablet to the phone.

Even in beta mode, Windows Hello has been working quickly and accurately to sign me in when I peer into the front-facing camera -- much of the time. It sometimes won't recognize you in certain lighting conditions, but there's a PIN for backup, and you can improve accuracy by running it again (and again) in various lighting scenarios so it creates a larger library of your authentic eyes. It even recognized me through my glasses, although you'll need to take them off on your setup scan.

Setup was easy, though the app is hard to find -- get to it through Accounts or Lock screen submenus. In settings, you set the allotted interval required to sign in this way, and you can use it to authorize changes to the phone and buy apps.

James Martin/CNET

Iris scanning isn't faster or easier than a fingerprint scan, and depends too much on lighting to get it right. It's a unique way to sign in, but not always better.

Windows 10 overall gives the Lumia 950 a much cleaner, more sophisticated look in every menu, from the settings array to the way you choose your themes. The app store looks great (but still doesn't have all I need -- see below), and there are a few extras lurking in the menus, including the ability to double-tap nav bar to turn off the screen (but not back on again). Cortana is a good voice-controlled personal assistant, and the ability to download offline maps is a real perk, especially when you're traveling in rural areas or cities abroad, and Wi-Fi and data are limited.

Yes, Google Maps has an offline feature that lets you save a specific map area that you already navigated to, for 30 days. The built-in Here maps app lets you download maps for an entire city or region to use any time, so long as you have enough space.

James Martin/CNET

There have always been hard-to-pinpoint things about the operating system that don't seem to work as smoothly as they should, because they seem tiny enough to ignore, like when I email a photo, the Outlook logo takes over the screen until the pictures send. Disturbing this threatens to cancel your email.

The Achilles' heel: App selection

If the new features of the operating system are decent (or at least promising), one aspect of buying a Windows phone remains a compromise at best, and a dealbreaker at worst: app selection.