I have both converters (the "Mutars"). The wide angel converter I use reglarely but the tele converter rarely. Both are of decent quality. A tele Rolleflex, of course, has a better lens in comparison to the Planar/Mutar.
If you use the Mutars you should stop down to f/8. This is the price you pay using an additional lens instead of an additional camera. The viewing mutar lens is slightly too small so you get dark corners (only at the ground glass of course).
I paid around 400€ (500$) each. I read there were produced not many, only around 1000. Mutars are equipped with different adapters for fitting at different bayonet types. It seems to exist adapters for BayI, BayII(45mm) BayII(42mm) and BayIII. It is said they work best with the 75/3,5 Planar.