To be honest the most noticeable difference i see with readyboost isn't the speed of loading - that remains about the same, it's more the fact that your HDD has far less time active.
Basically whenever something is written to the page file when using readyboost it is written to both the HDD and the USB stick (to avoid anything going wrong if the USB stick is removed while being accessed), however, when data is then loaded from the page file, the HDD remains idle when it would normally need to be accessed, thus halving the HDDs active time.
This has reduced system noise for me, but more importantly I'm sure it must have increased the lifetime of my HDD.
[EDIT] To make the most of readyboost, buy a fast USB data stick (like the sandisk titanium series - that has read speeds as much as double that of other brands) that ideally matches the capacity of your system ram, eg. i have 4gigs of ram so i need a 4gig usb stick.