How about the Pentax 50-200? Currently, I have a host of Sigma lenses, including the 10-20, 18-125 and 70-300, but I need a lens to complement the 18-125 when I'm shooting auto races, a lens with a longer than 125 reach, but less weight, and less reach, than the Sigma 70-300. An arthritic neck is part of the reason for the need for lower weight.
I've read and heard a lot of "should be" and "maybe is" and "someone told me" about the 50-200, probably because it is low cost. I'm truly curious to hear from a few people who have used the lens.