Well I went out and rented a 24-105 this weekend and was really fairly disappointed. I've been shooting with my Tamron 17-50 and after seeing a few drawbacks I thought I would venture for an L.
I loved the focal length, even on my crop body it was wide enough for most shots. However, I only had a couple of good sharp shots from the weekend! I only took one Tamron comparison and probably preferred the Tamron better due to richer colors. The 24-105 seemed to have a haze to it, didn't focus accurately, and overexposed most of my shots.
I'm assuming this is because it's a rental, but I don't think I would buy one without trying a different copy first. It was not an exciting experience. I think I had 3 photos that I thought, "wow that's worth the L" and I haven't had any issues with my bought new inexpensive lenses. I hope this is an odd occurence.