It'll be interesting to see the Alien prequel, Prometheus, as Aliens managed to buck the trend of sequels not being quite as good as the original.
Terminator IV, which I suppose is prequel, even though it's set after Terminator because of time travel shenaninghans, was fairly dire.
I don't see the Watchmen prequel really working, since we know how the stories have got to end and really as much of their back story as you need to know.
The Dune prequel novels were pretty bad as well.
Young Indiana Jones was never any better than OK and mildly diverting.
Enterprise might have worked if they hadn't felt so tied up with continuity (or recycled stories from ST:TOS and ST:TNG.
The only prequel I can think that works is The Magician's Nephew, which was CS Lewis's sixth Narnia book, though it's chronologically the first.