I picked up a 160GB X-25M G2 for $450 on Newegg the minute they were posted - I have it installed in an otherwise-maxed 17" non-unibody MacBook Pro.
It's an amazing and surprising difference, one that reminds me of my first dual-processor machine. It's not that your system feels faster when doing one thing (although it does), but that no matter how much you throw at it, it just doesn't slow down - a tremendous difference from the chug I experienced on the 250GB drive that was previously installed.
Easily the best $450 I've spent on an upgrade in a long, long time - far more effective than replacing my MBP would'be been.