I have 17" on my car. I love the look, and as everyone has said, the performance of the tires is fantastic. I have Firestone Firehawk SZ EPs, 275/40 in the rear, and they are awesome. They stick well and look great. The ride is stiff, but hey, it's supposed to be. I hear the wet traction is good, but since I have no roof, it'll never see rain.
I also autocross my car. Although my tires stick really well, I must say that since my last car had Kuhmo Victoracers, and street tire (regardless of size), can't hold a candle. I wish there was a way I could have a set of those too.
Steve