I rock SPDs on my fixie, use the same shoes (and similar pedals) for my XC bike. I like that I can have a pedal stroke or two to get that foot in on the SPDs. The rubber sole of the shoes will let me go a few rotations without clipping in if needed in traffic or whatever. Just make sure you get a shoe with a STIFF sole. BTW, I have Look's on my road bike.

There are a lot of threads that do not like SPDs for fixed, claim pop outs during skids. I have ner had a problem.