I couldn't justify spending an absolute fortune shelling out on a 512Gb SSD never mind what the 768Gb models will end up costing! I've just upgraded my 500Gb Momentus XT in my MBP 2010 because I was approaching 90% full for the XT 2 750Gb and it seems noticeably faster. For a single drive upgrade when you need large capacity and speed it's the best way to go for me.
One warning though - some users have found HFS+ problems with the XT2 on Lion 10.7.2 but I'm not one of them!