I've used my K20D with Sigma (non-APO) 70-300.
Quality is average to disappointing, depending on the situation. I was using the lens at a friend's wedding recently and very unhappy with the results. I guess available light goes a long way as to how happy you are with the cheap zoom option!
In full daylight, I've taken some good shots with the lens, but if you go pixel-peeping you will still be disappointed. I believe the 14.6MP sensor has better resolution than the Sigma 70-300 lens.
The short story is that I'm now saving towards the new Sigma 70-200 f2.8 SDM
If I can couple that with a 1.4x or 1.7x SDM-capable teleconverter I will be a very happy chappy indeed.