Local camera shop has a 1st gen used for $650. Should I jump all over this? I have never had a lens longer than 300mm. My 70-200mm 2.8 is so heavy I rarely use it. Hate to spend $650 on something just to watch it gather dust on my shelf. On the other hand, if I had it, maybe I would start shooting the birds in the woods behind my house, or wild life down at the lake.