I have a 2011 27" iMac that came with Lion. Upgraded to ML and now to Mavericks. While I never had any real issues with Mail or anything, the iMac did feel sluggish and I had the constant Spotlight searching. To be fair though my iMac was packed. I do a lot of video creation and editing and I had tons of old files on the hard drive. Still, with all of that on my drive Mountain Lion ran faster. I generally hate hitting the UPGRADE button because I end up with mixed results. I spent all week backing up my hard drive and completely erased (reformat) the iMac's drive. Installed Mavericks on an empty drive. It flies now and very smooth. Installing applications back on the drive one by one with zero issues. While I know Mavericks could still use a big software update my iMac feels like new again. Just to add: I know no one is going to respond to my post agreeing with my good experience because this place would rather hear nothing but negatives about Apple's products.