Good workflow capability and quite flexible in terms of customisation. Lots of industry-specific support, which is good if it exactly fits requirements (rare). Extensive data model which is good if it matches customer requirements.

Siebel is effectively a 'data model out of the box'. It often takes more time reverse-engineering what is there to see it it fits requirements, than it would take to define and configure it in more modern platforms like Salesforce.com. It is also not very stable or performant, and requires the purchase of hardware as it is an on-premise platform. It is also a significant upfront investment.

Since Oracle have released their Fusion applications and purchased RightNow CX, Siebel likely has a limited future. It is based on old, on-premise technology which has now been superseded by the aformentioned Oracle products and 'Cloud' players like Salesforce.com.

Scalable and pretty robust.

Interfaces and integration with other Oracle products is non-existent.

Fine if you wish to have standalone but do not buy for integration reasons

Not much to like... so old school.

This is not fun to use, requires a lot of care and feeding and if you want to change the way it works for your business get out your wallet.

Consider a modern CRM.

This was a good choice many years ago, however, there are now much better solutions for CRM nowadays. Many businesses have successfully implemented and run this solution, however, I consult in this space, and have not heard of a single customer in the past 5 years preparing to implement this.

Old software, old technology. Not worth considering as an option.

Consider Microsoft Dynamics or Salesforce.com CRM. These are much better more scalable options for small and large businesses alike.

Siebel used to be the big hitter for CRM. It is functionly rich, however take a lot of services to get setup

However the costs for implementing are far out weighed (by todays standards) compared to the benefits. A SaaS or even on premmise CRM can be setup and running in a lot less time

look at a modern CRM like SFDC, or even Microsoft or Sage for on premise.