I used to use it. The problem is that flash drives aren't designed to handle an extremely high number of writes. Most flash memory only handles about 10,000 write cycles before dying.
I used Readyboost on my last PC with a 2GB drive and every 6 months the drive would be dead. In the long run, it's cheaper to just buy more RAM.
If you do go for Readyboost, make sure you get double the amount of RAM in the flash drive. So if you have 2GB of RAM, get a 4GB drive.