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Just picked up a '12 MBA 13" with 4GB RAM a few hours ago and had to put ML on it myself, it wasn't already updated (weird) and it runs smooth, like another poster put it, like buttah.

I could also say that my '10 MBA 13" with 2GB RAM runs quite smooth itself.
 
My MBA 13" runs it fine, but that's SSD land. I upgraded my MBP 17" to 8GB shortly after I bought it (It was on Lion then) and It runs ML fine. I think if you can go to 8GN you are going to be better off.
 
I have a mid 2009 MBP 2.53 Ghz 4GB Ram after upgrading to ML , 3 days to be exact my laptop experienced some serious issues, apple diagnosed it as HD failure ( NOT) then said Logic Board ( NOT) , it was the Hard drive Cable
i had 250 GB that I've only used 90 GB pretty casual user, I was told the upgrade probably stressed the MBP how? they failed to tell me, anyways after getting that issue sorted out , I upgraded the RAM to 8 and this MBP is going pretty sold, I did notice a difference a lot smoother with 8GB running mountain lion
 
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