Thankfully, someone (ActiveWin) has finally gone out and done a review of USB Drives that are capable of being used by ReadyBoost.  (via Neowin)

The performance specs they quote may not seem like much (wow, a 8 second saving in booting Vista - wow!) - it’s actual app performance that makes a huge difference.

I used a 1GB Sandisk Extreme III Compact Flash card as a ReadyBoost cache last year, and it actually makes things much faster. Of course, I’m using a fairly decent system (Core 2 Duo 6600, with 2GB DDR2 in Dual Channel) - but application start performance, and general responsiveness were improved significantly.