First, it's free. Second, it's extremely compact and it runs as a background service. Third, it indexes, analyzes, and reports in ways you've never dreamed that you'd want. But you do -- want them, I mean. Example: Last year I received 11,138 emails (spam never makes it this far so it's not counted) from 1,589 unique senders. My "time to respond" is shortest on Mondays and in the morning. And my email and contacts are "grouped" in a very interesting fashion. The search is just flat out lightning fast and the interface deceptively simple.
Microsoft tried to buy the company (it's a San Jose start-up) but the founders didn't want to move to Redmond, so it's still independent. It's in public beta using an "invitation" system, but recently the site has been opening the beta periodically. Try www.Xobni.com to sign up --- if you find you need a direct invite, send me a note, as I have quite a few.