Just got mine off ebay via hong kong and So far so good - looks really good with my 135L at f4. Adds a little to the 16-35 without killing sharpness, and even is a nice addition to the 50L. Best of luck on getting one -
I bought a HK one (it is not bad for about $150) but I like the quality of the Tamron sp better.
I found the Sigma and Canon to be far superior to both, but you need to use some kind of restraint on the push pull zoom to not to go under 70mm for 28-300mm which works well for me now. I recommend a canon 1.4x and use 1.5 inch velcro tie around the zoom section to stop the push and pull zoom at about 70mm which will make the glass elements not to collide with each other at wider zoom.
only 2X teleplus pro 300