Made a quick test video about angle grinder diamond metal blade.

[Crusty]

I agree that abrasive cutoff wheels are the best choice for cutting metals. I prefer the Makita super thin wheels because they cut through steel quickly.

One application though in which the diamond wheels (and blades) shine is cutting rock. I have a Dewalt diamond rock cutting blade and it's just short of amazing how fast it cuts through granite countertop and it leaves a nice edge.