Always run spd's on anything other than a fun/playbike/bmx.
Used a set of the spd/flat shimano pedals from '97 till they died about 3 years ago, now use the DX spd's.
Clipping in and out is like second nature, and dont think about it at all, and agree with the comments about float. In fact, at traffic lights (yes, I am a stopper, not a runner), I dont even unclip, I just balance and make a quick getaway.
Only fell over once when clipped in, and that was on a roadie with LOOK's, in 1993
I get more knoee ache from a day spent on my V12's than I do in my spd's, but that could be down to the type of riding, or the riding position. either way tho, my feet can rotate MUCH less with Vans and a set of V12's
G