It's more useful for problems in the NP-Complete set, where you can write an algorithm that should take a very long time to perform, but takes very little time to check.

Take the 3-sat problem, for example. It takes a very long time for conventional algorithms to produce a correct answer; but, given a potential solution, it takes very little time to check for correctness. It's this area that quantum computers could shine in. Get a potential answer from the machine and check its accuracy very quickly. If wrong, try again.

[edit: if I misunderstand quantum computers, forgive me. This is just what I thought their allure was]