Mountain Lion on Intel Core 2 duo (2009 macbook pro)
I've installed Mountain Lion on a Macbook Pro 3,1 (2007). "Over the top", so no clean install.

Specs: 500 HDD, 2GB DDR2 RAM, 2.2 GHz C2D 

This is the oldest MBP which supports Lion. And it runs fine. No hiccups so far. It boots as fast as before (~1 min). It sleeps reliably. But I can't comment on the battery life yet.