Gary
We tried a couple and in short it depends what you want them to do for you! What we found was that the majority of them couldn't be slowed down enough with the variable speed so even with a skirt on the product was flying everywhere. Then when we did find one we could get to go slow enough it simply didn't have enough torque and therefore started smoking out the back end when any pressure was applied. The best ones seem to be the 4 or 6" but then they have a downfall over a larger area. In short it is difficult to maintain a consistent pressure, application of product and timing resulting in an uneven finish especially when honing & polishing with them.
We quoted a marble Kitchen work surface last night which involves two islands and two small side areas totalling around 7M2. The marble has completely lost its polished finish and is very deeply acid etched in a lot of places. This marble requires completely honing back up at least 4 and probably 5 times through the stages before it is ready to take a polished finish along with 17 Repairs. So essentially that will be 42M2!!!! Far too much with any hand machine IMHO. I quoted him two days to do this work using professional equipment but I reckon it would take at least 6 days using a hand machine.
The long and the short of it I suppose is:
It depends on what you want it for and what you expect of it.
I hope this helps you make a decision.
Regards
Kev Martin
Marble Life Ltd