I struggled for months and months over whether to buy this lens or the Canon 17-55 f/2.8 or the Canon 24-70 f/2.8. In the end I felt the 24-105 had a more useful range for me than either of the other two, and that was more important to me than the wider aperture. I also felt the 24-105 had a much better build quality than the 17-55.
I don't regret my decision at all. I'm very pleased with the results I get with this lens on my 30D. When I first got it I took it into the city for the day and got lots of good street and candid shots. It works well for landscapes too. I think this is the most versatile lens you can buy for a Canon body. This and a fast prime make the ideal light weight travel kit, in my opinion, which is what I was looking for.
I also wanted a lens that could be used on a FF camera, should I ever get one. With the 5D replacement just around the corner, that could happen sooner rather than later.