I recently brought one, but before I did I phoned around and found that I would be able to get it serviced, and some repairs but Canon themselves will no longer do those repairs. I decided that for the price I was paying I was happy to take this small risk given the amount of money that can be saved. I did decide however that I would pair it with a camera that allows micro adjust of the focus to get the most out of the wide aperture.
I brought the lens with a 2x converter so I now have a 600mm f5.6 for less then £1200 total cost.