Tried it once, my start-ups and shut downs were longer than usual and I couldn't tell any difference what so ever. I think it just puts the page file on the stick, I used a flash voyager which has a light that flickers.
I think readyboost is designed for either high-random read/write programs or for programs with a very slow primary drive. Neither of those tend to fall into our needs as enthusiasts. My F3 can do 5 times the speed of a normal USB flash drive, at least.
Ready boost was designed for slow computers, with slow HDD's and lack of RAM. Using ReadyBoost provide near instant access time, which is beneficial for such system when access the page file. The idea of this, is for those you can't or don't know how to upgrade their RAM and/or changing their HDD.