I have a black Lumia 820, been using it for 2 months and pretty satisfied.
Technically speaking, all the new WP8 Lumias are excelent phones. High-quality optics and photo technology, beautiful displays and minimalistic yet intrigant design. Never had a frame rate problem up to now, even with 3D games.
Nokia offers a wide variety of exclusive applications, and they constantly update them (in fact, I think those are the most common updates I get, and I use kinda lot of apps). If you're switching from another OS probably your experience would be suited best with a Nokia phone, comparing to HTC and Samsung, but that's just my opinion, other phones are really nice as well.
WP8 is growing constantly now, the app gap is closing but I feel obliged to make you notice that many famous apps aren't avaiable (Instagram in first place, although soon we'll have a way around its lack) and some officials aren't really good. Tho, lots of 3rd party apps are extremely nice and way better than officials on other OSs.
If you can pass over the current app gap, which I trust we'll close sooner or later, you'll like WP8 itself. Regarding phones, my choices has been over the 820 because the 920 was too big. For how much the big screen is a joy, it's a smartphone, not a tablet. The 820 is basically a smaller 920. The hardware is pretty much the same, slightly worse camera (still kicks *** tho) and display (just an AMOLED, still really nice imho), but you get features that for me are more important: microSD support, removable cover and removable battery. Lumia 820 is probably the more versatile.
This is just my opinion. Lumia 920 is one of the best smartphones ever made, and the new low-range devices like the 620 and 520 are really nice for that price, if you compare to the offer of Android junkie low price phones. I'd go over for Nokia, but do not forget there are also 8X and 8S phones from HTC which I can say only they have an appealing design, never had a chance to try one.
EDIT: forgot to add, the battery isn't really good on the 820. It will hardly get to the end of the day if you use it a lot. I gotta say that with locked screen and cool battery, it can go for more than 2 days

Remember to manage all your background process and you'll deal with the energy problem just fine. After the first month of usage, and my knowledge fully grown, I can go from 6:30 am to 18:40 pm without need of charging and without disabling connections.