I've never really felt that I learned much of anything by asking questions of potential employers. It either got a rehearsed B.S. line, or I made them uncomfortable and they were less likely to then hire me.

I do, however, come away with very clear opinions of employers, but this is based entirely upon their behavior during the interview. Not focusing on my answers (one guy brought a laptop to the interview and browsed his email while I was talking), asking keyword trivia, or generally coming across as a prick means I definitely don't want to work there.