I am using e-mail, contacts, and calendars, as I have been since I got MobileMe several years ago.
I have found Photostream to be convenient as well, since I take pictures from time to time (maybe 15-20 a month), and now I don't have to manually synch them or worry that if I lose my phone I will lose any pictures I took two weeks ago and forgot/was too lazy to synch up.
Though I guess I'd just lose anything since the last time I charged my phone at home, since there is no way to enable Photostream synching just being on Wi-Fi, which would be MUCH more convenient since I automatically connect to a few Wi-Fi networks, and as such wouldn't have to wait until I am charging my phone overnight to get pictures pushed up to the cloud.
Other than that, I am still looking forward to iTunes Match. That will allow me to keep about 3GB of music on my phone, the most important 3GB. Then the other 7-10GB of my library can be "on demand" if I ever decide to listen to it on the go.
Oh yeah, and synching bookmarks across all my devices is great too, but again, thats something I have been using since MobileMe.
I don't trust Document synching after reading about people's documents disappearing. Also, with the fact that you can't easily synch up with the desktop version of iWork, its kind of useless. I would find it to be much more interesting if I could work on a document at home, read it and make small edits on the go, and when I get back home the changes would be there.
Editing between just an iPhone and an iPad seems kind of limited and makes iWork in the cloud about 95% less useful than it could be. The fact that you risk losing your documents completely puts that up to around 99.9% useless.