Last year when Lion came out, I upgraded from SL on day one. Performance was terrible by comparison. Battery life on my MacBook Pro was shocking. A day later I did a clean install, and it solved all my problems. This year, I upgraded from Lion to ML, on day one, and although not terrible, performance wasn't the same. Today i've done a clean install, and it is faster and more stable than Snow Leopard or Lion ever was on a clean install. Take it as you want, but this is my experience. And I'd advise you to do it. (all of the above relates to a 2010 MacBook Pro 2.53 8GB) NB: I'm the kind of user who cleans as I go. I keep my Mac in good order constantly, so the upgrade process didn't on either occasion face a challenging, cruft engulfed OS.