icloud and multiple users on a single iMac

I am keen to use icloud at least for calendar and address book and email. My wife and I both have Apple id's and we both use the iMac, where all usage is under my user name / environment. I also run a Macbook Air that is also discrete from the iMac.

If I connect to icloud what happens to the (currently) disparate address books etc? Will moving to iCloud influence the way my wife and I use the iMac (e.g. would we have to always login to our own domains on the iMac?