This lens surprised me. I bought it used at a garage sale from a seller who couldn't give useful info about it's performance. Frankly, I bought it thinking it might be a usable telezoom for a student in our family who had expressed interest in photography and had bought an old 20D to that end.
But wait!, my first trial at 300mm, indoors in poor light rendered an amazingly sharp image of labels on cleaning products 15 ft away and the image stood up well to cropping out the center, maybe 10% of the overall frame. Hmmm.
Since then I've kept the lens and it gives very good results with both of my bodies, a 5D2 and a 1D2n. I feel awkward saying it in light of my original intent but this lens is too good to give away!
This one lives up to the description as a "sleeper" or "hidden gem" completely, IMO, and is well worth it's full retail price, again IMO.