All specs, all classes. If you're not an idiot you'll end up doing the majority of the raid's damage. If I go on my ~560 DK I end up literally doubling the next player, assuming they aren't doing the same. If you just hit an ability every GCD, even not playing your best, you'll probably still achieve top 3 no matter what. But that's LFR for you.
Otherwise, for actual raiding, DKs are viable but are in the lower end still. No worries unless you're going for world firsts, and even then it's a bit late for that.