i like leno's monologue, and the schtick after the monologue before the first guest comes out is also entertaining. as an interviewer, he started off extremely poor and often looked like he was in above his head. he's calmed down and matured over the years to where he conducts a decent interview now, but nothing that really is insightful or different. having said that, i like that he frequently finds someone's first commercial or acting gig and shows the clip.
conan's stand up is good, though he tends to wander all over due to his ADHD. his skits are terrific and i like his interviews. his helter skelter mannerisms seem to work most of the time.
kimmel is smooth, his stand up is excellent, and his sketch's work very well. as an interviewer, i think he's the best of the group. i started watching him a few months ago and have enjoyed what he does. i'll be watching him at 11:35, if and when i watch.
fallon's stand up routine is hard to watch, mostly because he isn't funny and so much of it is forced. it's been so poor that i haven't ever stuck around for what he does after that. i have no idea what kind of interviewer he is. i'll probably watch an episode or two of the tonight show just to see if he can change my mind, but i don't expect that he will. i think he's a terrible choice to replace Leno.