This is my lens of choice for night football and for indoor sports. It really does make a difference -- I'm shooting long after the f/2.8 teles are already put away, and making shots in the poorly-lit areas of the field that others can't capture.
This lens is extraordinarily sharp. Stopping it down doesn't improve the sharpness at all, as far as I can tell... maybe a pixel peeper can find a difference, but I can't. I only stop down to control depth of field; what other reason is there with this lens?
Similarly, I can't see the loss of sharpness with the addition of a Canon 1.4x teleconverter. Possibly a 1DsII would be able to measure find it, but I'm not quite there yet, so for me, the lens can be a 280/2.5 with no loss sharpness. Contrast drops a bit through the TC. Perhaps it's also slightly slower focusing with the TC, but I've not noticed that.
I shoot with a cropping sensor, so I can't say whether there's noticeable light falloff in the corners of the frame. Canon's MTF chart (see the 1992 version of Lens Work) does show a slight drop in sharpness and contrast, so perhaps I'm working in the sweet spot.
In fairness, it's a bit too big & heavy to handhold for very long. I tried to handhold for an entire football game, and it wasn't any fun. (by the 4th qtr, I was missing shots from resting my arms too much) Figure on using at least a monopod.
Also, if you desire a telephoto to take candids unnoticed, at the beach for instance, then forget this lens, go with f/2.8 and deal with the loss of 1+1/3 stops. This lens ALWAYS gets noticed, I call it 'the Eye of Sauron.' People a long way off can tell that you're pointing it at them.
In summary, it's a great lens for sports, very useful for portraits if you can't get close (parades?), with a fantastic ability to throw the background out of focus. f/2.8 with a stabilizer will sometimes get you in the same ballpark, but it'll never get you the shallow DOF... and for sports in low light, a stabilizer won't help you much, it's the subject blur that kills the 2.8s.
(Paid $3,600 from ChamCamera in the fall of 2005)