I hate the sound, heat and unreliability of traditional Hard Drives.
Carrying around a disk spinning at 10,000 rpm. Not a good idea.
Leo Laporte tried adding this flash drive to his MacBook.

Amazon – Corsair Storage Solutions – Solid state drive – 128 GB – internal – 2.5″ – SATA-300 US$330
It worked for Leo.
Installed a Corsair SSD into my unibody Macbook Pro. Boot and app launch times are lightning fast. Almost instant….
But others report problems.
There’s a 256 GB version for $760.
The price is far too high. But I hope Apple comes out with a reasonably priced flash drive model by the time I need to buy my next laptop.