Recent excursions to places with many birds that I wanted to photograph, plus meeting up with people with HUGE lenses, has got me hankering for something a bit better than my 250mm lens. I have looked around a bit, and the Sigma APO 150-500mm f5-6.3 DG OS HSM seems quite good and is the cheapest I have found so far (can get it for under $900).

I'd welcome comments and suggestions, but given that I want to keep it as cheap as possible, are there many (any) alternatives?