I have the M515 pedal on my road bike also. I also have a mtb/commuter/run around town bike that I use the shimano PDM324 that is dual sided for use with regular shoes. I use mtb shoes for the road bike and mtb, but recently I have purchaced a pair of road bike shoes. I like them both but if I had to chose one it would be the mtb shoes with the double sided pedals.
I also know lots of road bike riders that use egg beater pedals on their road bike!