I tried an experiment myself today (another chronic heel strike sufferer). Instead of using my mess bag, I took my smaller sling bag and an old Nashbar Townie basket that I had folded and left on the bike while it was on the bus rack.

Got to work, unfolded the pannier and put two of the upper layers in it. Then I started out okay, but soon overheated in my insulated jeans. Had to turn back around to the office for another reason anyway, so I changed into the track pants I had in the locker. Those jeans went into the pannier.... So long as I traveled in a straight line, I didn't really notice it. But cornering, dropping off of a curb, sprinting out of the saddle, I could feel how it was affecting my ride .

But, yeah, to those who can use panniers, good for you. As the for the rest of us that can't (or won't) use them-