I have the 7200rpm 500GB Segate Momentus XT and it's pretty good. Definitely not SSD speeds but it works well as a OS drive. Boot up and responsiveness is much, much faster than my previous laptop (i7 720qm, 4GB ram, 320GB WD Caviar Blue 7200rpm v.s. i7 2720QM, 8gb ram, 500GB Momentus XT).
It's no SSD but it is leaps and bounds faster than a conventional HDD, though it does depend on usage patterns. For example using it as a boot drive with drivers and things I use all the time like my internet browser and other programs will improve application start rates very well, but it's not magic. Just pure sequential copying of files and whatnot will not be sped up.
http://www.tomshardware.com/reviews/seagate-momentus-xt-hybrid-hard-drive-ssd,2638-4.html