I have multiple small strobes, and every one of them has a Sto-Fen Omni-Bounce. Two of the 580's for on-camera fill work, and 5 or 6 Sunpak 383 Super for small location lighting work. The Sto-Fen's are compact, easy to mount/unmount, and can be left on the flash all the time.
For on-camera fill, with the flash head pointed up at a 45-degree angle, the Sto-Fen provides terrific fill. It's high enough to avoid any red-eye, ever, and diffuse enough to cover a very wide lens in close on a group of people. Indoors, the bare-tube effect pops light off the ceiling and walls to effectively add more fill.
For off-camera lighting, I use them with clamps or on light stands, usually at a 45-degree up angle. With the Sto-Fen, the lights are wider (more coverage) and softer (fewer hard shadows). Shooting digital, I find I can light a fair-size lab or office with just these small lights. They are easy to hide, easy to clamp to a ceiling, etc. (I also carry snoots, homemade grids, etc., when I want a controlled area of light.)
The only time the Sto-Fens come off the flash is when I want to do bounce flash off the ceiling, but that's uncommon.