I use Mavericks on my main machine. It came with ML, but I don't think I stayed a month with it. Before that I used a 2010 MBA that came with SL, and which I use as my backup machine now. I kept updating this one, from Lion to ML, then to Mavericks, and finally decided to give it a shot with Yosemite.
Found the Yosemite UI a bit tiresome and the performance a bit subpar (maybe because it is a beta on an old system), so today I decided to reinstall its original SL OS on it, and boy, what a shot from the past! My first mac was a white macbook that came with Leopard, and the UI was just the same as SL.
I think a lot of people still use SL because of its nostalgic feeling. Leopard was released in late 2007 and till mid 2011 there were zero changes to the UI, almost four years of aesthetic stability. Then suddenly we began getting tweaks every year, doing away with aqua, launchpad, mission control, etc etc, and now we're getting a massive overhaul. I have to admit, having installed SL on my old machine, I copied all of its original wallpapers onto my main computer, just to have a bit of that "homey" sensation here too.