Siebel's most valuable feature is the web interface. As far as CRM's go, it's a very flexible solution, and we're able to customize the business rules with code and to integrate it with middleware applications to manage the rules externally. In our case, we've integrated all our BSS and OSS applications with Siebel using TIBCO v5.6.
The application itself contains complete functionalities needed to manage the interactions with clients -- quotes, opportunities, and offers. Also, we can register the interactions with our clients.
We have another instance of Siebel CRM (7.8) and in that case we have integrated also with Tibco BW 5.9 and we are currently using other products from Tibco for developing the BPM processes related (Tibco iDecisions as BPM).
We manage all of our client records and interactions currently with Siebel. We generate within Siebel all of our provisioning and billing orders and send it through our middleware to our billing and provisioning applications, as I mentioned above, we use mainly Tibco BW as our middleware platform.
In our particular case we do the integration using Tibco BW and in some cases we use SOA Suite.
I agree that Siebel provides a good repository for centralizing customer information.