Normally every alternative to Microsoft will offer the same (low)value proposition that Microsoft does. The result comes down to managing lock-in.
The book [link|http://www.inforules.com/|Information Rules] walks you through this in detail.
What is different now is that open source offers a viable alternative to the usual economics of lock-in. However it is only viable in some scenarios. (Namely either you are willing to take over your own maintainance issues, or you are confident that someone else will be willing to before you have to.) Figuring out whether you are in one of those is a non-trivial business decision. If you are not, then you could wind up stranded pretty badly.
(Unfortunately, of course, actual IT decisions tend to be made by people who understand none of this...)
Cheers,
Ben