I beleive the real reason MS introduced ReadyBoost is to save power in the labtop and notebooks and all mobile devices that would install vista. The speed perfromance is a byproduct of this.

Your drive will not be spinning burning valuable energy when you are using readyBoost. Only when the buffer is full will the drive kick in, this saves alot of battery life + increase performance because now you have a place where files are called up on the go, sort of;)

Now, you have to have special kind of USB drives (2.0+) fast usb link, or it will not do (could be wrong on that). My friend tried it wtih their USB and readyboost is not supported on thatone because they dont have a fast USB socket.