This lens wasn't even on my radar until recently; hadn't even considered it. I was originally looking at the 70-300 DO IS and happened upon the postive reviews for the non-DO version quite by accident...rather glad I did. I suspect it was due to half-remembered negative reviews of the 75-300 IS predecessor and the other 7x-300 lenses in the Canon range.
So, the lens. For a 'consumer' lens the 70-300 IS comes as a pleasant surprise. No, it isn't built like a tank but as a result it is light weight and portable. Build quality is about the same as my Canon 35 f2. I've not had the problem with rattling or lens creep that others have had, and this may be something to do with the fact the lens has a later model number. As mentioned above the fact that the front element rotates and extends is an annoyance, but not enough to put me off. It isn't as compact as the DO version but it isn't exceptionally large either.
Image quality is outstanding for a non 'L' lens. I was quite surprised at its abilities wide open at 300mm - much better than I expected. The IS does what it says on the tin; it is a virtual life-saver when it comes to handheld shots with low shutter speeds; I can get usable shots with 1/30 sec at 300mm. Having said this it does focus slowly and hunts somewhat in low light but then this isn't supposed to be a high performance piece of kit.
All in all, this is a great lens for those who want the reach (without compromising on image quality too much), but don't necessarily need a fast lens, solid build or quick AF. Highly Recommended.