We've already fully-reviewed Windows Phone 8, but using a Nokia Lumia phone can be a very different experience from the base OS. Nokia is aggressively adding its own software to the platform and just as aggressively courting third party developers for exclusives. The result is a slightly better experience on the Lumia than you can get on either HTC or Samsung.
In terms of third party apps, Nokia has a section of the store with entries like Groupon,Mirror's Edge (yes, it's old, but it's so good), ESPN, and a few others. But it's the first-party Nokia titles that really shine. Nokia Transit and Nokia Drive provide real options for navigation, while Nokia City Lens is an augmented reality app that makes for a good demo but isn't the most useful way to get around town. Nokia Music provides free streaming, but if you're on Windows Phone you really should be subscribing to Xbox Music Pass anyway.
The biggest addition is Nokia Maps, essentially a super-charged version of Bing Maps. The main feature it offers is the ability to save maps offline, but you also get indoor maps of popular venues, more augmented reality, and voice-guided turn-by-turn directions. Nokia has tweaked the interface a bit as well, with a swipe-up offering listed options for your location.
Unfortunately, the way that Nokia Maps is implemented on the Lumia 920 is imperfect. Although the original Maps app is no longer listed on the apps list, it's still hiding on the phone. Tapping an address link in Outlook launches it instead of launching Nokia Maps, which is likely to be a cause of genuine confusion for many users.
The AT&T version has a few AT&T apps pre-installed, but luckily they're just a long-press away from being jettisoned from your phone. Microsoft deserves credit for not allowing carriers to break the core experience with un-removable apps, something Google once promised it would do on Android but has failed to deliver on.
Even with all of Nokia's enhancements, the same issues that we mentioned in our Windows Phone 8 review still apply: there's a serious dearth of apps when compared to iOS and Android — and I'm not just talking raw numbers but popular apps that are table stakes on other platforms. Just as Internet Explorer 10 gets unfairly treated as a second-class citizen by many websites, developers are giving this platform short shrift. It's up to both Microsoft and Nokia as the standard bearers for the platform to make the case to those developers that Windows Phone 8 is worth their time, and unfortunately they're still working on that.