Some times they're good, most often they summer from the inherent problems associated with Prequelism.
The best kind of prequels are the ones that only influence/relate to the plot of the 'first' movie/film/book, and only deal with the characters in an indirect way.
A good example is Metal Gear Solid 3, in my eyes. It hardly deals with any characters featured directly in MGS1, the exception being Ocelot. Thing is, Kojima succeeded in making Ocelot less badass, but still somewhat believable. The game, also, doesn't screw up the mystique and mystery that surrounded the Snakeverse up to that point. Sadly, MGS4 did it with great force.
In any case, a problem with prequels is that it has to work in a framework already set. Also, the observant's imagination has already established/filled in the backstory. That's one of the reasons why the Star Wars prequels and Hannibal Rising were so 'shitty'.
On their own, they weren't that bad, but let's be honest here... In your imagination, Darth Vader was more of a true Shakespearean hero than a whiny ****, and the same goes (but less whiny, I guess) for young Hannibal.
Whatever the makers think of, and it doesn't help that their motivation is inspired by greed; because else they would have begun with the prequel, it sucks harder than what you yourself already thought of.
But, of course, there are always exceptions.
