I purchased this lens 4 months ago, with the intention of using it as my standard lens for taking shots at motorsport events. I am an events photographer and my day is generally spent 10 yards from the track, taking one "head on" shot at 135mm and then zooming back for the "side on" shot as the car comes past at 28mm.
The head on shot always seems to work fine, but when panning the IS just gets it wrong! about 10% of the time it just vibrates like crazy trying to stabilize and this causes me to miss critical shots.
People want to buy my photos straight away and having to tell them I have missed them is really hard...so most of my shots are now taken with the IS turned off and the ISO notched up on my 300D's. This is a shame because otherwise it is a good lens at a fairly good price.
I would recomend you get one if you do not intend to use it for fast moving objects. The IS works great in low light when pointed at stationary objects.