If getting AD Domain Services off of Windows will actually avoid the need to buy any Windows Server CALs, then it certainly makes the proposition more interesting. But, this presumes that nearly all the other services (file sharing, http, etc.) are also not Windows.
I have to buy user CALs for #1 and #2. One CAL gives me the right to connect that user to any of our file shares and to AD. IIRC, that cost when we last purchased was around $50/user.
We also have to buy a separate CAL for Exchange.
I would love to replace AD and our Windows file shares with Samba 4, provided that it was a stable, viable replacement which didn't add a lot of overhead for our admins. Exchange is a separate issue, and one we're currently exploring Zimbra as a possibility. It's early days yet there.
Almost everything else in our environment is Linux.
I have to say that Microsoft convincing businesses that they need to pay to connect to servers that they've already paid for is a shear piece of economic genius. Morally abject, but still brilliant financially none the less.
