I have it, but haven't had time to explore it enough. But it is very cool.
Let's say you have a collection of standard midi grooves. You've got a cool basic I-IV-V blues groove in E. If you want to use it in your song you can pitch it up or down, but it will still be the same chord progression, just in a different key.
EZ Keys lets you impose your own chord progression on that groove. If you want to change the B to a 7th chord, no problem. If you want to make it a minor chord progression, that's easy too. If you want it to play some jazzy chord that you wouldn't be able to call out to your band mates before the next chord comes up, you can do that too.
And if you want to pipe the midi output into another keyboard, it's not all that hard to do either.
My biggest question with it for the moment, is how often do the MIDI packs go on sale?