I've just finished downloading Mountain Lion. On first inspection everything seems to work well. It has taken about 4 1/2 hours to download and install, but I do have a fairly slow broadband here in rural Warwickshire. The process itself was very easy. Macworld.com has some good articles about how to prepare your machine for the big event which I found easy to follow: I don't think of myself as being very computer able, although I can generally use them well enough. This is my first MacBook Pro, and my first Apple upgrade process: I am very impressed.