I didn't have a chance to buy a wide angle lens before I left. I had wanted to get a 14mm or 15mm - or possibly one of the Tokina wide angle zooms (I don't like zoom lenses, but people speak quite highly of them and they come in attractive ranges). On top of that, due to a mixup in communication, we left our walkabout lens (28-135 IS) at home.
That said, I've used my 24mm f/2.8 exclusively and its worked out really well. We bought this lens on a whim before our trip to Guatemala, but it's proven to be a very reliable piece of equipment. There have been a few places I've wanted a few extra degrees of visibility and a couple situations where I had to bump it up to ISO 1600 - but with the 40D the images are still well within an acceptable noise level. The 24mm is a well respected lens and probably the best value wide angle lens Canon offers so I'm not too surprised - although the Tokina 10-25mm f/2.8 would certainly open up some additional options photographically.