If you're using per-user salting (and you should be) then you have to do n hash calculations, not just one calculation and n comparisons.

Might be feasible for a small number of users on an internal app, but unlikely to work on a big or public site.

I'd also be worried about information leakage. If you tell me that I can't use my password then I now know that it's used by someone else. If I work out who that is (which could be quite easy) then I can impersonate them.