I've commuted on:

Low-end flatbar cafe racer. It was pretty fun, light, nimble. In the end, a bike designed for speed sucks for real life. While carrying nothing (7lbs. or less) wasn't a big deal, the bike bogged down bad under any real loads.
3-speed English-style bike. IGH is decent, but I found that I just never actually felt the urge to bother with it unless I thought about it, a lot. The bike handled like a cantankerous 1 legged-crack...pirate...but with panniers on it, it was very stable. But slow. Did I mention the "not Lance's other bike?"
Single-speed mountain bike. Believe it or not, the stock tires did not cause that many issues with rolling resistance. Speed was decent. Rims held up to our border potholes (read foxholes) and generally craptacular road conditions and with a backpack FILLEd with over 30 lbs. of books, didn't show the slightest 'flex' under the load.
I'm currently riding:
Araya custom (circa 80s) with a 9-speed campy setup, and I've already buckled the rear wheel 3 times. I'm going to have to go in and replace the bearings, and I'm running campy hubs so that's not going to be pretty/cheap. I've gotta say its fun to ride in the drops at speed, but that lasts about 30 seconds before the next transvestite/light/car/foxhole/border patrol agent causes me to hop back up into the hoods. I also find that for all that sexy campy goodness, I never-ever shift.

I even had my Suburban break down last week, and use my Araya for grocery duty (big mistake) and still haven't found a need to shift while carrying a full load.

Bag in question is a Tinbuk2 backpack (http://www.altrec.com/timbuk-2/grubstake-pack). Best damn bag in the world. I NEVER leave home without it, even when I'm driving.