"IMS is defined by 3GPP/3GPP2 as a new core and service 'domain' that enables the convergence of data, speech and network technology over an IP-based infrastructure. It is the operator choice of control and service logic for primarily IP/packet-based person-to-person communication but also for person-to-content communication.
For users, IMS-based services will enable communications in a variety of modes - including voice, text, pictures and video, or any combination of these - in a highly personalized and secure way.
The most widely deployed application on IMS are: Instant Messaging, Presence, Push-To-Talk and Video Sharing. There is lackluster customer appeal so far.
IMS was supposed to help to be like cartilage between the legacy telecom architecture and IP-based next gen systems. It was supposed to reduce OPEX and enable new services quickly to the end users. Likely that has not all been worked out yet, although SKT and AT&T are working on it.
Informa's IMS Conference is next month. Here's one guy's preview.