So just how good is the 250-600 Zoom?

The short answer is - excellent.
A somewhat longer answer is - I don't know. I haven't been able to push it to the limits of what it can do.
Take a look at the following image (try to ignore the quantization artifacts in the dark areas: I was scanning from slides, and obviously need to tweak the settings a bit)
Closeup of BMW M3

It doesn't look too terrible, does it? The plane of sharpest focus is at the rear of the car, but you can see fine detail in most places.
Not too bad, considering the only lens support was a monopod, and the shutter speed was 1/750 second, which is not really fast enough to completely freeze the motion of the car.
Oh, and one other thing ...