Certainly not a replacment for adding more RAM..... It's controlled by Windows..... In my limited use of this feature it does boost opening programs..... I've seen it better used in Office based PC's where Outlooks been open and several word, excel, access programs databases etc so a heavy use business PC...
If you check task manager in Vista & Win 7 it caches most used programs ready for use so when you launch it most of it is already loaded if you look at the free section it's usually very low or at 0 already the Ready Boost feature adds to this also increases the cached part also.....
But just think as it's USB the transfer rates are fast but no where near RAM Speeds....
Now will it improve games....? Highly doubtful.