I'm going to just link to page 5 of the Tom's Hardware post which has a bunch of graphs of ReadyBoost and SuperFetch tested:
http://www.tomshardware.com/2007/01/31/windows-vista-superfetch-and-readyboostanalyzed/page5.html
The full article is here, which includes an explanation of what they both are meant to do, as well as how they tested them:
http://www.tomshardware.com/2007/01/31/windows-vista-superfetch-and-readyboostanalyzed/
What they end up showing is that in some circumstances, such as when you have too little main memory, this can be quite a good addition. It is good to note that it is not a flat caching system, it takes the speed and strengths (and weaknesses) of flash drives into account, and only caches those files which will access from a flash drive faster than a hard drive. It also doesn't use it to augment main memory in terms of raw size. If you are editing gigantic photos in Photoshop while playing World of Warcraft, the flash drive won't help you.
Think of it as a cache for frequently run applications, which load many small DLLs into memory on startup, for instance. It isn't treated like RAM or a swap file, but rather as a cache that can do some things must faster than a harddrive.
Personally, readyboost does very little for me. I have 2GB of RAM, and tend to play games. It might make my games start up slightly faster, but most of the data that games use isn't likely to cache well on a flash drive. It's a nifty feature, though, when you recognize what it is supposed to do and what it isn't supposed to do. It works, especially if you have too little memory to start with. Since I'm capable of adding more memory the "right" way, I don't need it. For the kind of person that won't ever open their computer, this is a neat way to allow them to squeeze a bit more performance from their machine.