Some do, but it's not recommended as it is often not fast enough at times and has a limited write lifespan.

I've recently acquired a 8gb 25mbsec Read, 18mbsecWrite Sandisk Cruzer Contour and will try. It's purpose was mostly for the Vista Readyboost, which takes max. 4gb, so 4gb is free. The Readyboost does work indeed and improves as it learns, particularly doing a quick series of boots.

Any particular reason you want to do this like carrying it around from home to office and back. Make sure to run it on the same OS like Windows for Windows, else you get in trouble. Size is not a topic, but if it works suggest to get good space as some project do swallow 1gb per job, but most at WCG are small, so 2 or 4gb should suffice, but again speed and robustness are the key criteria to succeed. Certainly set Write to Disk in BOINC to a long time. 999 seconds is the maximum delay.