I bought one for a trip to Vegas and ended up using it a few times. I still managed to carry my tripod around for most of the trip. If you want something to lash to your bag and usually don't carry your tripod, get it. The SLR Model steadied my 20D with 70-200 L about 30' over the busy streets of Vegas for a traffic blur shot. 'Course I didn't intend to let it fall if it wanted to, but the little bugger held on tight to a railing.
It takes a little playing to get used to the legs but I'm glad I have mine, even though I rarely use it.
Edit: Spend the money and get the Manfrotto 3229 quick release for it (or whatever monopod/small quick release that matches up to the heads you already own). HUGE time saver.
^^^^^^^
What John said. I have the SLR-Zoom and use it for throwing in a pack while hiking or skiing. The quick release is a good idea too, as attaching it to the camera is a real pain.
While no where near being as good as a good tripod, it does work quite well, I have used my 5D and 24-105 on it quite a few times. I find it works better, and is more stable, with the legs wrapped around something, then in a free standing tripod stance.
You've confirmed what I thought the uses of the Gorillpod would be - but I though it already had a quick release plate?

