I have used a 920 for about 7-8 months now. Mine's a pretty early developer model but from what I can tell the hardware is the production hardware, it's just missing a bunch of accessories. Nokia threw everything and the kitchen sink at this phone (like wireless charging) so it is a little bulky and heavy, but no worse than other smart phones of this size.
Battery life is pretty decent for a smart phone, at least with my usage - I don't make many calls, it's mostly email and using it as a podcast playback device. I can get an easy 2 days minimum out of a charge that way.
I love the screen, but the successors - 925 and 928 - are supposed to have an even better screen.
GMail works fine with it, as far as I remember the built-in email client even has the GMail settings preset in it. The email client itself is a little on the barebones side compared to, say, K9 Mail on Android or ibisMail on iOS but it compares OK with the "stock" clients. Oh, and it integrates really well with a corporate Exchange server if you're so afflicted.
I use the phone on T-Mobile, it happily does HDSPA+ on their network, browsing speed is fast and voice quality is better than what I'm used to from my Android phone on Verizon.
Camera is pretty impressive for a cell camera. It's got decent but noisy low-light capabilities and is great overall as a snapshot camera when I don't have the digital SLR on me (ie, most of the time).
App availability is not as good as iOS or Android by a long shot, but so far I managed to find everything I needed, which is admittedly not that much - BoA Internet banking, Gasbuddy, Skype, eBay, Amazon mobile, a weather forecast app and FaceBook are all available. As is Angry Birds .
One killer feature for me is the navigation app - it works really, really well (Nokia owns Navteq IIRC) and pretty much has replaced my Garmin standalone for most car usage. Admittedly the app can be a little fragile (I had to reinstall it recently to get it working properly again, some things don't appear to change) but it's got everything I want including speed warnings and most importantly it's an offline nav app so you're not incurring data charges to use the navigation as long as you downloaded the maps at home. Plus it's easy to read on the pretty big screen.
It also integrates reasonably well with a Mac - the MS Windows Phone app for Mac OS does allow you to sync the phone with iTunes so that's how I'm getting music and podcasts on the phone. Syncs pretty well both ways, too, only "missing" feature is that it doesn't transcode to lower bitrates on demand like iTunes can do when you sync an iPhone. As I tend to store the music in Apple Lossless format that makes for not very much music on the phone but what is on the phone sounds very good through good headphones.
Downsides - no card slot for additional memory, battery isn't user replaceable and at least with my car charger it still runs down the battery when using the navigation app. That might be more a feature of my car charger, though.
Oh, and you need a "microsoft account" to use it, aka a hotmal/outlook.com address.
We have to upgrade/side grade our main phones because we're about to move to T-Mobile and if I could get a 920/925/928 from T-Mobile I'd seriously consider that over getting another iPhone.