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