Now that the "normal" brands went to very (but excellent) expensive teles, Sigma has more room to enter the game. I say enter, because previous models were bad (I know, I know), and I have tried them all (well, them meant the 500 and the 800) but the "monster" 300-800...
The others don't see much interest for me, as I'm well served with the Nikon counterparts.
What is striking is, after all of the hypes for the art line, namely 35, 50, I see lots of them in second hand market...