I went today, as well. I got there kind of late, so I was worried, but I was (pleasantly) surprised by how crowded it wasn't. I played Ridge Racer, the MGS3 demo, DOA, Street Fighter 4, Resident Evil and Kid Icarus.
It was a little hard for me to get used to the 3D effect, but that may have been due to the lighting? The overhead lighting was reflecting off of the screens, which could have caused the problem, I guess. I was most impressed with Resident Evil and MGS.
The fighters felt really difficult to play with the 3D effect on and I turned it off on both DOA and SF pretty quick. I noticed some vertical screen tearing on DOA, which was kind of disappointing, but it seemed to play fine.
Kid Icarus looked and played great. To the person who asked above, Kid Icarus was attached to the podium and you had to sit down to play it. All of the other games allowed you to pick up the 3DS and play with it standing up. Kid Icarus might be a bit difficult to play with 3D on just holding it with one hand.
Ridge Racer looked alright. Not sure if I'd play it with the 3D on or off, though. I was kind of worried about it, as the other two Nintendo Ridge Racer games(64 and DS) have been garbage, but it felt like a proper entry. Not super pleased about the price, though.
I went in pretty hyped about the 3D effect and I left feeling kind of mixed on it. The sweet spot on the 3D is pretty small, I think, and when I left my eyes were feeling pretty tired. I got the feeling it's the sort of thing that would look really amazing in a dimly lit room, but if I were using it outdoors or on a train or something, I'd likely just turn the 3D off completely. In addition to that, it just seems like a handicap in certain games, like fighters.
Also, I wanted to play Pilotwings but I had to leave to catch a train. :[
I will be trying to get my hands on one Day 1, however, so I feel pretty positive about it on the whole.