Another vote for brief! The last three weddings I was at, the best man launched into a lengthy, rambling speech like it was some kind of acting audition or something. Say something sincere and keep it short.
True story: I was the best man at a wedding one time where, because of general disorganization at the reception, I never even gave the toast I had prepared. Not a single person noticed until hours later when we were getting ready to leave, so don't get all worked up about it.