well, in my humble opinion, the F50 gt is an okay car, but the only time i use it is when there are really long straights, and then its only b/c everybody and their mother doesn't seem to think this game is about fun..not ALWAYS wining. Bbut to follow the majority here, for me it really isn't all that hard to handle, one just has to remember it has a lot of power turning the rear tires and drive acourdingly....
My personal fav A class car is the Mclarren, i've been a fan of it for years, it holds corners like none other and still has a good pick-up.
For B class, i don't like to follow the crowd and use only the Enzo, i actually avoid it in place of the GT90, the single coolest (and heaviest) car in the game.