Seeing as the Golden Master is now out, I thought a thread that colllected all of the reviews would be helpful to those who are thinking of upgrading. I personally love Mountain Lion, it seems a lot snappier than its predecessor. It makes a great accompaniment to the iPad and iPhone (Messages and Notification Center), and feels like a solid upgrade, well worth paying for. I'm running mine on a mid 2009 MPB and have experienced no issues. I'd advise an upgrade to anyone who thinks that their Mac is slow under Lion, as 10.8 builds solidly on the foundations of the last OS.