Well, you don't have to use the fake-money ones. Intrade and Betfair have always seemed perfectly serviceable to me, and they're real money prediction markets.

On a related point, fake money could actually be good. There's less motivation to bet what you really truly think, but not wagering real money means you can make trades on just about everything in that market - you aren't so practically or mentally constrained. You're more likely to actually play, or play more.

(Suppose I don't have $500 to spare or would prefer not to risk $500 I do have? Should I not test myself at all?)