There's an article with some benchmarks on Adaptec's MaxIQ system here
You have to use x25-e's for cache though it won't let you enable it on x25-m's
I'd test it myself but I only have x25-m's
Tom's conclusion is about what you'd expect - so long as the working set is under the cache size the performance is great. The real question is what size is the working set of your average desktop, gaming, server, etc? It's hard to say.