OCS is great if you can afford it. Remember that people wanting the phone to just act like an old phone system may not go for it. It really thinks in a new paradygm. If you cannot afford on premise OCS checkout hosted OCS. I've written an article going over some Hosted options.
You could take a peek at pbxnsip for Windows. This is a very stable IPPBX for Windows. pbxnsip has a ton features to match older, traditional pbx's and integrates to Exchange and OCS. It does not have AD integration. At the moment 3CX will import aD users but nothing more.
I think Adtran has a windows product (forget name) that does have AD integration if that is a deal killer. My impression is that it was a little lean on features (no blf, etc)
On 3CX: I've written a book on 3CX and spent multiple 1000's of posts helping people with it and 3CX has User Interface nailed. But to my disappointment my personal experience has been that it isnt the "set and forget" level I'd expected from a pbx. Our consultancy uses & recommends pbxnsip for Windows PBX. It has all those features traditional pbx'ers throw at you and runs like a rock.
So you can pick: Easy to use, lighter on features, less rock solid, cheaper= 3CX. Every feature you can think of, extremely rock solid, bigger learning curve, not bargain basement=pbxnsip. Move into the future and lay out some cash=OCS.