As far as companions go, I LOVE Cass for her sassy, hard drinking attitude and one liners. I have known a few girls like that ;P. And of course, the fact that she's Cassidy's daughter makes her quite the nice reference to Fallout 2.
Boone is a little boring to me, but maybe I haven't found his quest yet, if he has one.
Veronica is a decent character but there is kind of an arrogance about her that I don't like.
Gannon, in my opinion, is a pretty stereotypical liberal douche-bag homosexual with a chip on his shoulder, so I don't like him that much. Hey, I don't care who you sleep with as long as you don't pathetically self depricate, play some kind of victim and lie to me all at the same time. Also, I wouldn't want to go into battle with someone who snarkily says "Uh, ok I guess I'll try" and rolls his eyes every time shit hits the fan.
The others I either haven't found yet or don't have enough experience with to form an opinion.
Yes man is, of course, the popular favorite and for good reason. Being voiced by the great Dave Foley helps.
I thought Ceaser was a great character though so far I've only met him the one time in his camp. He has a cool "will to power" kind of philosophy and some excellent dialogue and voice acting. Also, I like that he doesn't personally take the whole roman shtick too seriously despite being its progenator.
Benny is another good one - at first I thought he might be kind of a standard sniveling villian but by the time I killed him I actually had some respect for the guy.
The ranger chief was very well acted but his dialogue made him seem like kind some kind of half-mystical nitwit at times.
Nothing jumps to mind about another character that I really dislike, but I guess that's a good thing.