While I haven't used the lens myself, I've read enough about it (and similar lenses) to give this one a RENTAL =) Seriously, any lens, and I mean ANY lens with such a large zoom factor will be about quantity over quality. There's no physically possible way for a lens with a 10x zoom to be super sharp and dimensionally accurate at all (or even any, as often happens) focal lengths. Just look at the Canon 18-200... Yeah, it's a lot cheaper and all that, but the 28-300 will also give you distrortion across the board, and won't be crisp at the default aperture...
Another downside is the push-pull design. No, really, the best lenses Canon makes are either non-zoomers (obviously), or lenses that are completely sealed and don't extend, such as the 70-200 series. Any lens that extends and retracts on a constant basis will be more prone to physical damage, as well as sucking up dust, etc, and don't let the BIG L fool you.
I'd choose any 70-200 over the 28-300 any day. If you really need all the zoom lengths at your disposal at all times, just get a second body with a wide angle lens.
The 28-300 just isn't worth the money, IMHO.