If you're looking for some of the most dramatic improvements in daily task performance on your computer, you'll want to switch out your traditional hard drive to a solid state drive (think of it as a huge internal flash thumb drive). You'll also want to check your bank account, because SSD drives are still quite expensive. Seagate is offering an excellent compromise, a hybrid of both traditional and dependable 7200 RPM spindle technology, married with an 8GB flash memory buffer, the best of both two worlds (speed and dependability at a reasonable price).
We've long ditched the optical drive, utilizing the space for a secondary internal drive. The Momentus XT Solid State Hybrid is fast and spacious.
We've dropped one into our MacBook Pro (as a secondary internal sidekick to our smaller boot 120GB SSD) and haven't thought about it since; quiet, reliable, and returns fast boot times and file transfer speeds. Installation is ridiculously easy, and the payoff is night and day performance.
The 5 year warranty is impressive too, reflecting the higher shock resistance construction of these units (note laptop users) and the dual storage technology's decrease of wear and tear which eventually leads to drive failure. The Momentus XT Hybrid may be the best non-compromise, compromise you can make, whether you're a OS X or Windows user, especially considering the entry price. Highly recommended!
If you want to geek out further to performance benchmarks and specific comparisons to other solid state and platter drive solutions, I highly recommend Benchmark Review's 10 page review.