The 820 being my first WP8 device, and I got it on launch day. Of course, there were a LOT of problems plaguing WP8 then, auto-reboots, and what have you. Nokia refused to exchange a new device for me, and denied the auto-rebooting problem as existent. Pissed the heck outta me.
Didn't manage to stick with it for a 2 whole month before I switched to the 8X.
The camera was horrible. (because it's pre-Portico) and as mentioned, the shells. I like to switch them every 2 days or so, and it was a pain in the @ss to remove, broke a few of my nails. And most importantly, it didn't have a 'retina' display. My eyes are used to PPIs above 300 and I just couldn't get used to it. Using the indigo theme, I would notice that there was an extra pixel of red and blue to the left and right of the tile. My eyes are pretty sensitive to light, so maybe I'm just nitpicking, but the display was the most horrible part of the entire device.
It's still a great device nonetheless, just that it doesn't suit some users.
(I have the 920 now!)