If you have MBPR, the ML is worth far more than $20 (although you get it for free if you buy one). it makes it into a new computer almost They really goofed for even shipping that thing with Lion.
I've only got a 2009 13-incher, but I don't doubt it. They probably should have announced it at WWDC but with a "ships in July" kind of thing. One month later, but massively improved first impressions sounds like a good trade.
That said, one question for those who've used the betas...in Safari, is there a way to remove or switch the Google search thing with the history? I dread having to hit the down button several times just to get to my commonly used websites.