The last time I took out my MTB with its Mallets and plain shoes, it really wasn't too bad, even for the hour or two that I was riding.

I also ride my road bike often with plain shoes. It's got road pedals, though (Time RXS). I almost prefer them to CB pedals because their mechanism doesn't poke into the shoe... buuuut, of course, the platform is a lot smaller, and standing to pedal should be done carefully.