The lens represents the forefront of technology within a consumer budget. There are better lenses, and I would buy one if I had the money to spare, but this range of lenses allows me to follow my chosen hobby and achieve results I can be proud of.
For the money, this is quite a capable lens, but I have been spoilt by the sharpness of another recent purchase, the Sigma 10-20mm EX DC HSM.
A bit of un unfair comparison, as the Sigma is the equivalent of L glass, and wide angle lenses tend to be sharper than tele-zooms.
The great thing about the Canon is the 3 stops of help the IS gives you in lower light situations, but it's still no substitute for poor technique.
Need to give it a more thorough work-out to check the sharpness in different situations when the light gets better, but would recommend this lens if you can't quite stretch to L glass, it's a lot better than any similar lens for the money.
Great all-rounder, semi-pro optics, large focal range with the benefit of the Image Stabilizing action, so great for taking on your travels & on the move.
I do a lot of wildlife photography so it suits my needs well, as the void between quality semi-pro lenses & Canon L Series lenses is still vast.
This is one of the best non-L series telephoto lenses. Don't compare this with the other cheapie 70-300mm or 75-300mm lenses. It is a major upgrade from the 75-300mm IS that it replaced.
The lens has an ultra-low dispersion lens element (that is normally reserved for the L series). But only has the lesser non-ring USM which means that the autofocus isn't the fastest, and you can't manually override it without switching to manual. But in all it's pretty reasonable. Although the list price is £400, it's available online for under £350, or on ebay for less (although not a lot less, because it's quite sought after). Until the end of June 2008, Canon have a £40 rebate on this lens (you must provide the bar-code, and receipt) so double check with your retailer.
It's not too bad for lens flare, but I'd always recommend a lens hood. The ET-65B is a circular hood (because the front element rotates during focusing). But it is a bayonet mount, and will store back-to-front on the lens when not on the camera.
Summary: A good focal length range, reasonable build and optical quality, the IS helps to make up for the slow aperture.