I've been using the ReadyBoost feature since it came available in Vista. My desktop computer was originally a Vista 64-bit system with 12GB RAM. You wouldn't think it would be beneficial, but it was. As memory-hogging as Vista is, my computer never crashed once I added the 4GB ReadyBoost drive. It did before that. Now I've upgraded to Win7 Pro 64-bit and it's even nicer to work with.

I also use this feature on my laptops, two with Vista, one with 7, all 32-bit so they max out at 4GB RAM internally. The added memory really makes a difference. I didn't actually notice it until I removed the flash drive from one of them and it almost felt like it was hanging when opening docs/webpages, printing, etc.

Definitely worth trying for as cheap as flash drives are nowadays!