Don - I just finished evaluating a copy of the 28-300L via the CPS loan program. I primarily tested this lens on a 5d2 using auto ISO and a monopod at a major botanical gardens - so my subjects were moving a bit slower than you may experience.
This lens is a beast and a workout. That said, for an 11x zoom, IQ is impressive and IS is amazing. Auto ISO on the 5D2 (which I am just learning - as I recently obtained both the 5D2 & 1D3) had me shooting mostly at shutter speed = < 1 / focal length.
In the 5 or so hours I worked this lens, I had > 400 captures and a keeper rate > 90%. This despite the goofy auto ISO settings (re inappropriate shutter speeds). I had many 300mm shots captured at 1/30 or slower - AND the keeper rate was very high. Talk about good IS (most shots were on a monopod).
It is what it is. Not sure that it's sharper than my 70-200mm f4L IS or my 100L or 180L macros, but it's not bad. Probably the best all purpose zoom that I've used - at least as far as IQ.
That said, after working it and lugging it for most of the day, I felt very beat-up (physically) and yearned for my 70D with its oft-disparaged EF-S 18-200mm.
Another issue that I found annoying: the rubber on the underside of the tripod collar stubbornly stuck to the lens barrel - making it impossible to loosen the tripod collar to flip the camera into portrait mode. Thus, if mounted to a monopod or tripod, one could setup and shoot in portrait mode or landscape mode, but not both. Thus, the larger files of the 5D2 - shot in landscape mode - could be cropped to portrait mode in post processing - but I wouldn't do so with smaller file sizes (as on a 5Dc or 1D3).
My summary - tentative as I'm still reviewing these results:
Pros: nice IQ, solid and fast (mostly) AF, impressive IS (Canon claims 4 stops and that's pretty close).
Cons: $$$ - wow!!! (the refurb price just increased ~$300 today, not that any refurbs are actually available), massive - made of lead?, extremely conspicuous (really stands out in a sea of photographers, ugh.).
All in all, I was impressed with the images but not so much with the massive bulk of this beast, and the construction or build quality was the poorest I have EVER experienced in an L lens. No excuse for the tripod collar. It was a piece of junk. I found that the lens tightening ring was constantly annoying - as it stuck to the focus ring in such a way that one needed 2 hands to tighten or un-tighten this ring. Sadly, I don't have 3 or 4 hands to secure equipment and make such adjustments on the fly.
My purpose was to see if this lens could be used for higher IQ than my crop mounted 18-200. The answer seems to be a heavily qualified yes, but the qualifications are severe.