I ran with SPDs for the first time last night. Noticed a 2-3mph positive difference in my usual cruising speed as well as ability to accelerate a lot faster without nearly as much strain as I would've on my platforms. I may need to tweak them a touch, but it felt just fine if not weird to be locked into the bike, especially the weird subconscious desire to backpedal to slow down (I have a freewheel..) so maybe it's time to think about making the old Schwinn non-susp. MTB I have into a fixie for some fun.