I've installed the Seagate Momentus XT hybrid drive in my MacBook Pro and so far I'm really impressed with the performance. This drive has 4GB of flash in addition to 500MB of regular magnetic storage. It automatically fills the flash with sectors that are used most often, such as files for booting, etc.

The net and real-world results for me seem to be quite improved boot and login times, and near-instant launching of apps I always have open, like Path Finder, Safari, Mail and Evernote. It's especially impressive for Mail and Safari which I don't have starting up automatically and may quite and restart a few times per week.

So now I'm going to wait for the new round of SSD and then install a 128GB model as a new boot drive, with the Seagate becoming my secondary and primarily data drive.