This and a Tokina 12-24mm ultra wide angle lens are my travelling companions everywhere I go. The Tokina is wonderful for landscape/seascape shots, the Canon 70-300mm for getting in nice and close for sharp detail. I've also used it very successfully for portraiture (allows you distance, so no need to crowd your model) and for fine art photography subjects. It's an absolute boon for travelling, being fairly light, and the image quality is consistently high every time. Thanks to the Image Stabilisation, hand-held shots, even at the full telephoto 300mm range of things, are plenty sharp enough and despite its relatively small aperture, the IS ensures the lens performs well in fairly dingy light too - providing you know how to set your camera up right. Okay, so the build isn't L class, but then you're not paying L Class money either. For a very useful and extensive zoom range, image quality, and convenience, this lens is invaluable and excellent value for money.

Used this with my Canon eos 450 and it is excellent !

I bought it mainly for taking photos at the theme parks and my children's school plays and sports days. I managed to get some great shots of my children on the rides at Drayton Manor Park after my wife missed them all with her compact because her digital camera (a 2 year old Panasonic Lumix digital compact) simply couldn't keep up with the speed and focus. The USM and IS combination worked so quickly and so well - got some great photos of my kids on the fast moving rides - I captured the expressions on their faces as they enjoyed the thrill of the rides, rather than being limited to the usual static photos which I would otherwise have taken at the beginning or end of the rides with my compact on previous occasions (Just be aware though, the camera does become very heavy with the zoom lens attached, but for the quality of photos, it was well worth it !)

I'll only review the IS aspect of this lens and make one other comment since I have only had it 24 hours and have yet to go out and about with it.

Image Stabilization:
Having solved the problem I comment on below, I hooked this brand new lens up to my 400D and started shooting objects at maximum focal length across the room using available light only. No flash. I took two frames of each, one with IS on, and the other with IS off. Shutter speed was 1/10 - 1/20 second in each case. Yes, 1/10 - 1/20 second and I was using 300mm focal length, hand held. Those frames taken with IS off were blurred beyond any semblance of use. Those with IS on were pin sharp. I was blown away. Truly. One of the frames was of a toy car on a shelf, the car occupying the whole photo. Naturally, depth of field was short - which added to impact - but for the chosen focus area every spec of dust was clearly visible. No lens will completely compensate for bad technique, but I proved that with good techique (in this case how to hold equipment and relax with low shutter speeds) the realms of expectation can be dramatically extended.

Comment:
On first attaching this lens to my 400D the camera would occasionally "hang" mid shot and need the battery removing to reset it; no buttons would operate, the screen went out, and bizarrely the camera's power LED stayed on even when the camera was switched off. I was sure I had a duff lens, but found help on the web saying "FULLY recharge your battery first, and then try again before sending the lens back". They were right. My battery (actually two in a BG-E3 grip) were not showing flat according to the camera but recharging them had the lens working perfectly. The reason for this behaviour is that the IS system demands much more power than a normal lens. Nothing to stop you buying this awesome piece of kit, but something to be aware of.