I had one for a short while and thought it was surprisingly good.
I only sold it to pay for a 70-200 VR, but in all honestly I wish I'd kept the 28-300 VC.
I can't really give you a comparison in Canon terms, but in Nikon terms, it has better VR (VC) than the Nikkor 70-300 VR, and optically its quite close too, and it has an extra 42mm on the wide end.
The VC really is excellent, 300mm is just rock steady even at stupidly low shutter speeds.
I think the best way to describe the lens is "suprising" because given the range it coverage it's a lot better than you'd think.