I am always on the lookout for a nice KP90DC or S&W 4566. I have shot several .45's, but have never owned one. I don't think you could go wrong with a p90, they are very well built and rugged(it's a Ruger, after all), and are supposed to be extremely accurate.
I would reverse what stuart9534 said, they are extremely rugged and reliable and well built, but accuracy was OK. The one P90 I have fired was at least battlefield accurate though but not a tackdriver like a Blackhawk or 1911. If you're buying it for protection, go right ahead I would trust one with my life.