Crown PCC-160's are really good. I've tried about 2 or 3 other kinds of floor mics, and they have all failed. Take some foam from your next change of a stage light bulb, and cut it in to the shape of the PCC-160, and tape the whole deal down. Sounds nice, just make sure that you're far enough behind the mains. Side fills aren't usually the problem, it's the overhead cluster that doesn't like the apron positioning, but we still get very little feedback with the PCC 160's, if we get any.
We've rented them in the past, but we just sent in a purchase order for three (only $700!)