I have a pheonix 2x teleconverter that was given to me by a friend along with some other gear. Ive used it once (let me emphasize once because I took it off almost immediately) with a sigma 28-200 f/3.5-5.6. Admittedly I haven't given it much of a chance but I felt that the loss in quality and functionality of the lense was not worth the extra focal length but thats just a personal stance. In addition, the piece feels cheaply made with almost a plasticy feel. I wouldnt count on it to be super reliable and hold up to any abuse.
However, if money isnt an issue then the price perfect if you're simply looking to experiment. Heres a short consumer review I found with a few sample pictures. I quote from the bottom of the review "it is almost unsuable with a 2X." He compares the lense to more of a telescope than a camera lense.
http://forums.dpreview.com/forums/read.asp?forum=1020&message=18284841Anyway, I think Dave and Prinothcat said everything else worth mentioning so no need to expand on anything they wrote. Just decide for youself and good luck.