Once you login for the first time, you can set your location, timezone and even add your photo.
If you are synchronizing your emails (@me.com), contacts, calendar, you can see all the updates. You can send emails from your iCloud account as well.
Find your iPhone is something interesting, you can do the same things from another iOS device
If you are using Pages, Keynote or numbers from your iOS device, you can now save the documents into your iCloud account.
Once you start using your iPhone / iPad after your upgrade to iOS 5 which is expected in few hours from now, you can synchronize your emails contracts, documents, backups, photo or you can store almost everything, but the problem is you have only 5G as free, or you will have to upgrade your account, but again the rates are very much reasonable.
One thing that I am wondering about is the photostream, I have tested that on my iOS devices, but for my MAC, I guess there would be an iPhoto upgrade very soon to support the photostream feature.