Here's a photo of the 16X9 0.7X HDV7X1 wide angle adapter mounted on a Tokina AT-X 20-35mm f2.8 zoom lens. This rig is about as front heavy as I can comfortably handle.
Cheap wide-angle adapters, regardless of how they're branded, will turn your sharp, high-resolution prime into a cheap wide-angle lens. Even the best wide-angle adapters will have barrel distortion, high susceptibility to lens flare and require a larger lens shade if not a full-scale matte box.
Some of the highest quality wide angle adapters are made by Century Optics and 16X9 Inc. These are priced in the $500+ range and are quite heavy. I've used a 16X9 0.7X HDV7X1 72mm with numerous primes and its quality is indeed very good:
While this particular 16x9 model is no longer in production, it can be found used on occasion. 16X9 currently makes a variety of wide angle adapters in various magnification ratios. I would not recommend an adapter with less than four internal lens elements or with a magnification ratio wider than 0.6X.