You may find some tips in the thread from Nagesh which takes a look at architecture and business elements of IMS
https://communities.cisco.com/thread/3614?tstart=0
Interesting that you state IMS is next generation network. I see IP Next Generation Network as a superset. To simplify a bit, IMS is a subset (or intersection) to leverage SIP for certain session oriented services, and offer standard interfaces for multi-vendor interoperability.
One tip, don't get caught into hearing IMS and thinking of it as a noun, something you go out and buy, turn on, then operate at profit. Some messaging makes IMS sound that simple. It isn't. Pure architects and real contractors always find the devil is in the details when remodelling - be it a building or a network.
Another tip may be to look at the vision for IMS, which is to enable interoperability between devices for certain roles. If a vendor offers you IMS as a shrink wrapped package, you may be getting painted into a corner, where interoperability isn't the real motive (How suitable is the shrink wrapped deal to extension for non-IMS elements of the operating network? If something cracks under the explosive data growth of the Mobile Internet Revolution, are you free to go into the "shrink wrap" and start replacing specific elements in areas where weakness was uncovered during operations?)
Cisco has products with interfaces to comply with the role they play within IMS and TISPAN deployments, while at the same time maintaining existing network interfaces, as well as providing next generation interfaces to comply with Web 2.0, video collaboration, efficient Content distribution, etc.
Cisco must embrace interoperability. Look at our history built from a foundation of interoperability in multi-protocol routing and IP, a history of many many acquisitions of products and companys to join the Cisco family. It remains in our interest to be prepared for new additions to the family, to be prepared to interoperate, especially in Mobile Internet.
I'd also like to hear more from the community on experiences for how "pure" or how "thorough" existing IMS deployments are? What services or roles are providers finding IMS benefit in practice?
