I purchased a Lumia 900 on 4/8. Previous phones include Nokia N900 (still "running" MeeGo...) and HTC G2. So far the Lumia experience has been good.
- Fluid, smooth and intuitive UI
- Majority of everyday apps I use are native MS/Nokia, or good quality 3rd party (UC Browser, Nav, Google Voice, social networking, email, music player)
- Excellent Nokia Drive navigation, probably best nav app I've used. Very quick GPS lock, nice UI while driving, good voice navigation
- Good battery life. 16 hours of moderate/heavy use (not always on LTE though), with still juice left.
- For me the home screen tiles work. IMO they are a nice compromise between an icon and widget, obviously this boils down to preference. Then again, if you load up your home screen with tiles, you've effectively made your home screen an app menu, just with super large icons...
- Display works well in daylight
- VKB is ok. Pretty quick swapping from portrait to landscape mode, but like with any touch keys, nothing beats the real keyboard. Pleased to find my native language as a quick switch option while typing.
- LTE speeds so far have been good (7-8 mbps download in West L.A.; good/great 4G-speeds in Miami)
- "Multitasking". Obviously nowhere near what the N900 offered, but in my daily use I find it similar to Android.
- Last but not least, though this one seems to be an ignored feature of a modern smartphone: fantastic call quality -- mind you this on AT&T's famously shitty network.
- Browser options (for now): IE9 is ok, not great, but I'd like to see Firefox and Opera Mobile as options. UC Browser as an IE9 variant offers some welcomed tweaks.
- App menu: minor gripe in my use, but if you load your device with apps, the single file scrolling can become a real nuisance. Not that I found Android's App Drawer with 4-5 pages (or a cube/wheel/cylinder...) that much better. I use the KISS principle, worked on Android, works on WP7.
- Battery (no percentage) and time displayed only on main screen, not when in app menu.
- Display: while it works well outdoors, the screen res. does show its numbers, especially while browsing. Pixellation is apparent on the browser while zoomed out, after you pinch zoom in the text/font looks ok -- for some reason I'd think this would be the other way around.
- Camera: Not bad, but not excellent either. It seems like my N900 takes as good pictures as Lumia 900, though this is subjective as I am not a photo-pro. So, the cam is a minor disappointment.
- Integration with Zune. Only reason I've used Zune was to do a firmware upgrade. But, signing up with Microsoft/Apple, you kinda know that this is the shit you have to put up with.
- Lack of specific apps. I haven't found an app yet that I couldn't live without -- i.e. I've modified my phone usage, like I had to do with the N900 -- but Android app market makes life much more fun if you have the time and energy to read about, test, and install/uninstall apps.
- The non-upgradeability to WP8. This one's going to bring some suckage. Though I have to admit, I wasn't even thinking about it when I bought the Lumia 900 (ok, so I got a refund for the purchase price, thanks Nokia!). So I guess I shouldn't complain after all. And it's not like my 2-yr old G2 was getting any love from HTC/T-Mo (ICS Beta on it, ran like, a beta...).
I have no idea how the app devel process varies between WP7, Android and iOS, but from an enduser, albeit not a superuser, experience, IMO Lumia 900 works well. I've dumbed down my device requirements, but so far I'm liking it. When I want to dig into an OS, I'll just continue tweaking my Arch with OpenBox :)