I upgraded to Mountain Lion from Snow Leopard. I use 13" Macbook Pro 2010. So far I'm quite pleased with the new OSX. It is a significant improvement over the SL in terms of overal functionality.
Here is what I enjoy the most: 1) integration with iCloud, 2) full screen mode, 3) notification center, 4) ability lto run the latest XCode.
However I noticed few problems (so far): 1) Safari crashes and freezes the ML -it is useless in its curent version so I switched to Chrome for now, 2) the notebook's CPU runs few degrees C warmer, 3) the battery drain is higher and the notebook does't last on single battery charge as long as it used to when I eas running Snow Leopard on it.
I think that the Mountain Lion has a great potential but it still needs some updates and bug fixes for it to really shine.