I have got this lens on my EOS 50 and LOVE IT !!!!! Two of my mates have got them as well and love them. For the price a great lens.
But i was at the PMA expo here in melb at the W/E and Sigma have now got the 50-500 (Bigma) with OS. It was around the 2k mark and it may will be worth having a look at.
A lot of people say buy the L series, i have taken 1000's of aircraft images with this lens. And sold a few as well, donated heaps to Aviation Museums and RAAF Squadrons and they did not give a **** that i used a sigma lens on a canon body. So get what you can afford.
Good Luck.