Circle pad: this thing is awesome, much better than the PSP 'nub' thing.
D-pad: kind of clicky and it takes some time to get used to it being so low, but still pretty good. Would have liked better a less clicky, Wii remote-like d-pad.
Select/Home/Start buttons: awkward placement and feel, but functional I guess.
L/R: again, too clicky. But work well enough.
3D: I found that I enjoyed much better with the slider set at the middle, really nice, I'm looking forward to it in future games. Still, I'll play most of the time with the 3D off because of the battery.
Original DS games: when you play in 'full screen' mode it does look blurry, I personally prefer playing with the original resolution which sadly means you get a noticeably smaller screen with black borders around it (think playing GBA games in your DS), but the image remains sharp. The circle pad works great for DS games, but I still prefer the d-pad.
The built-in software is nice, especially the AR cards and the 3D camera, but I won't be using it a lot after I get some games, really. Overall feels like a really nice handheld. Even if I'm stuck playing DS games on it for the time being
