I sold my MB-D80 recently. It's built as solidly as the camera, and worked perfectly. Really nice balance on the camera. When I sold it, it held a decent resale value.
The cheap knock-offs are probably built on reverse engineered moulds, and are probably just as tough. But, I'm of the opinion... that why spend £500 on a camera from a prestigious marque and then sully it with a cheap aftermarket addon, when the original is only £50 more [small change compared to some of the lenses you've been looking at, I'm sure...]?
I wouldn't buy a Porsche then stick some Halfords alloys on it. Of course, someone'll be along to say it looks and works well, and you've saved £50. Other opinions may exist..