Just picked up a 750GB Seagate Momentus XT SSHD.
It's brilliant. A couple of reboots and my system starts up just as fast as it did with an SSD. It does seem a little sluggish at first - on the initial boot-up I was thinking WTF?, despite expecting it to be slower to begin with - but it gets quicker as it learns your habits. Rarely used apps do take longer, but the HDD component of the XT is a 7200rpm laptop drive and it seems pretty nippy.
I really can't emphasize enough how little difference there is in day to day performance between my system now and what it was like with a Crucial M4 installed. Wholeheartedly recommended! I'm already thinking about getting some more for the rest of the household PCs, whereas before, SSD pricing put it out of reach.