"Have used this to cut through 2 steel RSJs, each at 10mm thick. It went through the plates (with patience) using a Ryobi 18V cordless drill. I predrilled the RSJs with a 3mm drill first, to make life easier, then used the holesaw with a 6mm mandrel. Holesaw still seems sharp enough to do a good few more cuts - can't really say more than that. I'd only give it 5* if it lasted 15 holes through steel but I haven't tested it.
Although I'm a DIY'er, imho, I've got as much experience as many 'trades people'."