I've known a few celebrities, particularly in sports. All my "celebrity jerks" stories are secondhand. I've heard a few about Barry Bonds in his early days in Pittsburgh (shocking, I know).
Don't know if this counts as a celebrity. I got to work with some people in the Commissioner's office a few years back, and can tell you that at least one prominent member of Bud's inner circle is a complete jerk. I'll leave his name out, in case I caught him on a bad couple of days.
When I interned for a club, I came across a couple young "bonus babies" who were immature. But nothing to the level of warranting the "jerk" label. Maybe I got lucky. Generally speaking, it was my observation that rookie ball players who are about 19 or so are more mature and friendly than college kids of the same age, even at "elite" colleges.
Worshipping athletes is dumb, but I give the benefit of the doubt that they're at least decent people until I hear otherwise.