Well, when i worked for Acer Corporation, i used Siebel. It was a very good system on that time (7 years ago) by now, it is one of the best CRM software packages out there.
Now with Oracle's adquisition of PeopleSoft and Siebel things will change and SAP should start to worry.
The deal was closed with a 5.85 Billion dollars. lol, just that?
More details here.
Also check siebel's website.