I have been using the 24-105L since it was released. It's proven to be a great lens on the 1Ds2 and 1Ds3. As Mark said earlier, it's weaker side is the 70-105mm range, but it's still good for walk-around purposes. At 35mm I think it resolving power is excellent, and its 24mm performance is better in the corners than the 24-70L. I recently update the Canon 24-105L review (
here) and the 24-105L gallery is pretty up-to-date as well (
here).
I like using the 24-105L in conjunction with the Leica M9 and 50mm Summilux ASPH (and other M primes). Basically, the Canon kit uses zooms, and the M kit provides the primes.
The 24-105L's IS is also excellent. I can shoot 105mm at 1/30th and have very sharp images. When the light starts to fall, the IS really helps, or when using a CPL and losing 1 to 1.33 stops of light and still trying to stop down to F8 or F11.