Agree with all of you - the fixie (early 80's Fuji conversion) is great as a commuter (at least for *my* commute - relatively flat; worst hill is the 1st 1/2 mile leaving home, then a few overpasses) vs. my mtb with slicks. (I prefer the MTB when the weather may be nasty or I just want to take it easy) Even an old 10-speed improves when you remove all the unecessary (and heavy) components. It feels and *is* faster. I have better gearing on my road bike, of course, but I don't worry about leaving the bike during the day, mounting racks, etc. I find I pay more attention to my pedaling, road conditions, and scenery as I don't think about which gear I should be using.
My most noticable "problem" is actually going from road bike to fixie - I have to remind my legs I CAN'T COAST (the attempt to pause to clip in the other pedal usually takes care of that).