Apple updated its iOS version of the iWork suite with new file management and iPhone & iPod touch user interfaces across the board. I currently only have Pages, so I installed the newly universal app on my iPhone 4 to give it a whirl.
Let's just say it's bittersweet.
Pages on the iPhone looks very nice and operates well given the constraint on screen real estate. That said, I'm not going to write a novel on my iPhone. Heck, I'm not going to write a full page document on my iPhone unless the situation were critical.
This all being said, Pages will be handy for the occasional edit. And that's where I think this app fits on the iPhone. Creating very brief documents to send out on the go, or making quick edits and sending them out.
I like Pages on the ipad and find it very usable for brief document creation. The iPhone interface should be a nice companion.
I can't speak for Keynote or Numbers, but just from the screenshots, Keynote on the iPhone looks like it could be handy in the same capacity as Pages. The Numbers screenshots look downright atrocious on the iPhone.
For $10 each, the apps are a bargain, especially if you find you only need one or two of them.