I've been using Xobni for a couple of days and I'm really impressed. I'm a statistics junkie about stuff that I do, so email statistics are just fantastic. The search engine is nice, but the forwarding and reply controls from the Xobni bar make that much more valuable. It does duplicate some of the functionality that was introduced in Outlook 2007, so it you already are using 2007, it isn't as mind-blowing as it would be on 2003.
That said, statistics rule, therefore Xobni rules. People are ranked by the number of emails sent to and received from them, thought it seems that sent counts more than received. The Xobni Analytics feature is fantastic. It gives views on email by time of day and some other neat layers that I can't remember right now. Good stuff.
Outlook contact information is integrated, but Global Address List contact information, like phone numbers, is not integrated. There may be a way, but I haven't found it yet.
Give it a try if you haven't already.