I have a 3X in K-mount. Its not actually horrible, but I certainly wouldn't spend any huge amounts of money on one. As mentioned above, they work MUCH better with macro shots or anything really that's less than infinity shooting.
With infinity shots, lets just say "I hope you enjoy seeing every lick of dust hiding on that sensor of yours." Diffraction is also nasty (even wide open as even an f5.6 lens is going to shoot up to f11 (if I did the math right) and have an extra glass element thrown in. Adding those three stops means the things are only really useful with lenses that have wider apertures, and are decent at those wide apertures, because any little flaw or defect in either the TC or the lens is going to become noticeable, fast. Since you're also disconnecting the lens from the body by shoving the TC (which will likely have no contacts) in the middle, you're killing the ability to stop down meter as well. Its wide open or nothing.
Heres my particular beastie.
And here are some shots taken with it with the Bower 35mm f/1.4.
This is the same stack of change shot at MFD with the same lens, sans the 3X TC.
Note that the Samyang 35 isn't a macro lens, it just has a decent MFD of about a foot. I really should try one of the pseudo-macros (really, they're more close focus than anything) I have with this thing one of these days.