A man dressed in a nice suit or in a nice shirt and trousers is really attractive to me. It says that he takes care of himself, has got a career, etc. I'm much more likely to take him seriously than if he were in a college guy teeshirt and jeans.
I prefer guys in really laid back clothing, like jeans and a t-shirt. If I see just any guy in a suit, I don't think twice about it. However, if it's a guy I'm already attracted to (like a crush or boyfriend) then a suit is hot.
I think when a guy is dressed up he looks really nice, but I'm also really attracted to the hott sexy guy who just got out of the gym or off the field. My favorite of all is the country boy with his cutoff shirt blue jeans and boots.
I'm very careful about suits. On one hand, you've got an old fashioned, interesting young guy, which is hot, and on the other hand you've got the business clerk commuter high flier watching his personal assistant grind on his stain glass birds eye view new york city map limited edition company made coffee table. Who's a total d***, every time. And its usually the latter - either that or some goof putting on a suit to look snazzy. Very rarely is it gonna be a cool guy. I used to rock out the suits, though, hahaha.