I have an Airport Express hooked into my whole house audio system and it rocks. Only problem has been that you need to go to the computer to control what music is playing throughout the house.
I'd say that using the new Remote app in the iPhone will be a great hit for me. Since it is a network remote, I can use it anywhere in the house!
If you don't have a whole house audio system to plug into, you can always buy multiple Airport Express units and get a set up amplified speakers for each Express and pipe the music wherever you need it. Of course, you will need multiple computers as well running iTunes if you want to listen to something different in each Airport Express. But, if you don't mind having only one source, you can pipe iTunes to whichever Airport Express units you want (or any/all of them).