I have SPD pedals on one bike. I was having numb toe problems with some toe clips and shoes and clipless seemed to fix that. The negative is the bike with SPD pedals only gets ridden on a planned ride. It's nice to just hop on with tennis shoes and go and the clipless pedals sort of put a damper on that.

I can't say that I notice a real performance boost with them, they mostly improved my comfort level. I wear road shoes, they are impossible to walk in but have really nice vents to keep your feet cool. Clipless pedals are nice, especially if you have multiple bikes and don't always have to wear special shoes.