The search function alone makes this a worthwhile download as it is much faster and more comprehensive than Outlook’s native feature. Xobni also adds some functionality by presenting email as Gmail-like threaded conversations, organizing attachments, finding connections between senders and recipients, and by extracting and making visible on contacts’ profiles useful information like phone numbers.
They recently moved out of beta with version 1.7 and made a number of tweaks so it doesn't drag Outlook performance down during message switching. They've also ironed out some bugs and are apparently looking to expand onto other platforms as well. Best of all, it is free and it doesn’t get in the way. You can download the latest version of Xobni here and share your thought in the comments section.