Well, I believe that most computer with enough amount of RAM to satisfy Vista (e.g. 4 GB) will not gain a noticeable performance benefit from this feature. Here's some of the benchmark report :
http://www.anandtech.com/systems/sho...spx?i=2917&p=6
From the above article we can see that even having a 1 GB RAM without ReadyBoost is much faster, and since the price for RAM is pretty cheap nowadays, it's simply more efficient to get your computer more RAM, rather than relying on ReadyBoost.
ReadyBoost was actually intended to helps performance of computer with limited memory (e.g. 512 MB) at the time of Vista launch time, so people wouldn't need to upgrade their hardware to using Vista (which probably generates more sale for Microsoft). Indeed, it seems that their implementation is getting better after time, but still, since nowdays people usually have more than 1 GB, and the good performance of the new Windows 7, I don't think this feature will do much for most of us.
Moreover, for those people who have multiple operating system, and use Windows less of the time (like me), relying on such OS specific feature to have a performance boost is definitely a bad idea. And from my experience, people that running Vista with 512 MB usually won't open CS4 with 14 images...