The comics I've read (the first three hardcovers that are set post-series) captured the voices of the characters and the feel of the series spot on. The likenesses weren't bad, either, though they weren't as close as the character voices.
I'm kind of curious about the two D'Argo hardcovers that I've seen. Since they aren't set post-series, and since I'm short on cash, I haven't picked them up. But if they're any good, I might give them a shot at some point. Anyone read them?