Like SuperFetch. I think ReadyBoost needs some time to settle and if you're just testing clean builds every other week or all you're doing is loading games and streaming large video files and stuff you probably won't see any difference other than a bit faster startup and app load times but on certain kinds of heavy use multimedia, programming type workstations I think it might even have it's place even if you have a boatload of ram and a fast HDD.. that is at least until OCZ drops the Colossus SSDs in a few weeks (and I cry when I see the initial prices.. )
I don't think of readyboost as extra, slow ram, but more like a little ghetto SSD drive for small files that are accessed often, and frees up the hdd to work on the big stuff instead of scrambling around for all these little files that take it relatively long to access. I'm pretty amazed at how fast GIMP x64 loads on my machine now for example. The thumb drives may not be able to compete with a hard drive for data rate but the access time is much lower (my cruzer 8gb says 0.5ms in HDTune, while my Seagate 1tb hdd is 12.6ms) I don't really claim to understand how it works at all but it seems to work for me, just my 2c.