Alright, in the interests of full disclosure, everyone should realize what they are getting. Yes, it's "tiny" (thus the Tiny Turbine name). Nevertheless, it is a pretty awesome and finely detailed piece. Here it is with several other popular spinners.
search
As for spinning surface, flatter appears to be better. The low clearance doesn't lend itself to highly concave surfaces like a lens. I need a very steady hand not to grind it into my 20cm focus lenses. Otherwise, it's a great little spinner.