These kinds of converters are always going to cause some loss of image quality. That being said, if you just need an occasional telephoto or wide-angle lens, then they may serve some function for you.
However, remember that used lenses are often available, and may be great buys if you are willing to take a chance or if you buy from an established store that will warranty their used lenses. Sometimes, these stores also have eBay outlets.
I've bought a lot of lenses on eBay, including some new and used ones, and I've generally had good luck. For example, I've bought 3 used Canon EF 50mm f/1.8 lenses (two as gifts, not to replace failed ones) and all of them have been like-new in condition and utility.
Swede
My Gallery