I've had mine for years and have loved using this lens. Sure it's not as sharp as a 300 2.8L but it's a 10X zoom!!
I originally bought it for shooting ski racing and only wanting a single lens to bring on the hill in a LowePro Orion AW fanny/daypack with my then A2e or 1n film body.
The lens has turned in fantastic results in such bright conditions. And, has also proven to be very useful for those longer reach portraits at the telephoto end with the slight softness actually facilitating the results.
With the film bodies, I've never had any problem with dust nor weather effects getting into the lens and the absence of IS hasn't been much of a problem either. It's all about technique I guess.
Haven't even seriously considered replacing it with the 28-300L IS lens, as this one works so well. Now that I'm shooting with a 1DMKIIn the film bodies don't get any use, and yeah there's a 1.3 crop but it's only a field of view crop not a magnification multiplier, so the composition of the shots with the 1DMKIIn's 100% viewfinder effectively eliminates any problems with the crop factor.
I almost never use the tripod collar, and haven't found handholding the lens to be too much of a problem at the shorter focal lengths. At longer lenghts, a stable position, railing, tree or some sort of support can be helpful if shutter speed isn't high.
This glass is not supposed to be compared to the L Series primes and any such comparision will achieve nothing.
Is the lens great? Yes. Am I delighted to own it? Yes. Would I buy one again? Yes. If I did not own one would I buy the 20-300 L IS instead? Not sure!