My unibody Macbook Pro 15" shipped with a Hitachi 320GB 5400RPM drive. Nothing wrong with it, but not quite as zippy as I wanted.
So, I ordered a Seagate ST9500420AS, which is a 500GB 7200RPM drive. Nifty! Except, its been on back-order since February (yes, its now June). I finally got it into my grubby hands, used SuperDuper to migrate all the data, and then compared the xbench results. Data attached.
Summary; its worth the upgrade!
In terms of impractical, bogus numbers, startup time has dropped by 7 seconds, and application launching time by 28 seconds. Real-world usage shows a subtle (but important!) increase in I/O which means you see the beachball of death less frequently.
PS. Came across fseventer, a great file system utility. Worth downloading (credit to Macworld for the find).