If you haven't used a computer with an SSD you don't know what you are missing.  They are incredible fast.  A couple months ago I installed a Corsair 120 GB Force series into my nearly 4 year old Macbook Pro.  It feels like a new computer.  Apps start faster on the laptop then my 1 year old, top-of-the-line Mac Pro at work.  I will never buy another computer without a SSD.  Now that computer are starting to come with thunderbolt ports you can have an external hard drive to hold any overages from the smaller SSDs (unless you have a ton of money to buy a big one) without worring about speed.

Update: Ben Brooks just mention something similar in his latest post.