How low a price are we talking? If it's something you can easily afford in case it's not that great, go for it. Heck, if it's $300-400 I'd probably take it, as long as the glass was clean and in good shape. Consider it an opportunity to write a full review of it for us
Beside missing the accessories (sold with nothing but the caps), it's in very good condition. Glass is clean, & autofocus works. Looking w/ zoom on 5Dmk2 LCD & quick look on computer screen, photos look good.
[I make a point of keeping everything including the box, it seems a lot of used lenses are sold to shops with nothing but the caps.]
I'm buying the lens from a store, and the price is U.S.$500 + the local equivalent of sales tax. For comparison, new lenses are often sold around here for 5%-10% more than U.S. prices (before taxes).
A review? I would upload a few photos (I'm planning on nature & architecture, will consider requests) taken w/ 5Dmk2 (anyone interested in photos taken with 600D?), and leave writing accompanying text of both the lens & my photography skills to the critics.