Just my 2pence worth, I've got one of the challenge grinders, yes Very cheap from argos. well having spent the last few days cutting out the floor of a rotten disco and welding in a new one the grinder saw some action...........
Well All I can say is, This should only be used IF THERE AIN'T a BETTER OPtion...my hands were fizzing from the vibrations after only a few mins use. And Loud!!!! wtf!
I'LL be buying a repacement set of spaners for my old Hitatchi, asap!!! 20 yrs old and still a better machine altogether.
I also found that I was getting some problems with my hands and fingers after just a few minutes use, I guess if I used it for too long I could end up with "whitefinger". I've started to use it with my welding gloves and that seems to reduce the vibrations somewhat. The vibration maybe caused by the cheapo discs included with the grinder or more probably because the output shaft on it is poorly balanced.
I only bought mine for hobby use and not for professional use, although if I was looking to buy one professionally I'd have spent a small fortune on a reliable and trusted make.
I didn't find it that loud actually, but I'm using ear plugs with mine and don't do that much grinding, I'm also working outside with it. I guess if your working inside and under a car, that would increase the overall noise level...