I have one for my Canon. I keep it in my camera bag because its a lot easier to carry this about at 600g than it is to carry about my Sigma 100-300 f4 at 1600g, and I have it for those occassions my 24-105L doesn't cut the mustard.
The 24-105L is an excellent lens and superior to the 28-300 at every focal length up to 106mm! Whilst you can zoom with your feet to some extent, you know what I mean!
I took it to Brands Hatch once for motor racing (my big Sigma hadn't arrived in time) and was pleasantly surprised by its performance.