For a lot of people it'll depend on talent. If you're good enough for "my" team then I'll root for you (see: Bonds, Barry re: San Francisco). For me, it'll depend on what kind of jerk he is. I couldn't bring myself to root for Roger Clemens when he was on the Yanks. It was always a "Hope they win 11-10 tonight" situation. I can think of other guys I wouldn't root for (LoDuca, Pierzynski... what is it about catchers?), but there are jerks that I will like on my team. I was all about some Shockey when he was a Giant. Can't stand him now. I'm not sure what the distinction is. Maybe I like jerks that are goofs?
Anyway - just to waste away the weekend pondering something. If Bryce Harper