I've got a 2GB SD card dedicated to ReadyBoost on my laptop. I don't notice a speed difference while using 'normal/standard' functions, but for things like gaming and PhotoShop there's a noticeable speed increase, though it's not much.
I've also NEVER had any issues with my laptop crashing when removing the card while running, nor did this happen when I was using a flash drive...on either 7 or Vista. I believe ReadyBoost is actually setup in a way to prevent such problems.
thread has quite a bit of information about ReadyBoost and 7.