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For now, iCloud tabs only work if you have another mac with ML. Once iOS 6 is here, we'll be able to use it with our other devices!
 
The start up time has definitely increased after I installed Mountain lion. But, that is because I did not do a clean install. Plus this is the third OS X version that I've installed on my computer, a 2009 13 inch MacBook Pro. When I finally upgrade my current hard drive to a larger hard drive, then it might be faster. That's the only complaint I have so far.

Everything else is faster. I also noticed that Safari uses less RAM than it used to. That I'm very happy with. When I use Safari on Snow Leopard, it took up a lot of RAM and it would slow down very often. Now it's Firefox that hogs up more RAM. I'm also happy with more integrated iCloud into Mountain lion. I wish Lion got that right, but hey better late than never.
 
There are many new features that I like in ML, especially iMessage and Notification. Safari has cooler animations too.

However, raw performance wise, I would still say Lion is better. I did a clean install on my rMBP and found that start up and shut down times increased slightly (after doing PRAM and SMC reset or it was worse!) Scrolling lag is more obvious than Lion (after the recent Safari update), and if you don't believe it... try Facebook, zoom it in and start scrolling. :)
 
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