I had a Kingston Data Traveler 1 gig thumb drive lying around, so I decided to try out Ready Boost. I stuck it in, and Vista asked if I'd like to use it for Ready Boost. The thumb drive has to have sufficient enough speed in order to be used; Vista will tell you if it isn't fast enough. I have 2 gigs of DDR2 system RAM. My PC does seem maybe a little faster now. If I had less system RAM, then the speed difference would be much greater.
I've seen a lot of reviews that say that Ready Boost actually hurts system performance but I haven't seen any since SP1 was released and I know they made some changes to it. I've been using it since I first started using Vista about a year and a half ago and I think it makes a slight difference but it's only really worth it if you have a flash drive laying around that your not using, don't buy one just for that.