Yes, Xobni is brilliant.
Have you ever tried sorting an Outlook inbox by conversation? Of course Outlook goes into a thrash while it prepares the view. Then when it has finished, it does not work right. It has a limited view of what a conversation is, based on the email title. It does not show your sent items, unless you sort them into the same folder. In fact, it is more frustrating than useful, which is why I never use it.
Xobni (the name is inbox reversed) does this right. When you select an email, a panel shows your previous emails from that person, with your replies, which you can read without changing the focus from the message you are attending to. It is based on an index together with some simple analytics. Who else has appeared in the cc list on emails from this person? Where are their messages? What is the sender’s phone number? All of this information is shown automatically; no need to hit confusing menus like Arrange By or Current View.
There’s also a search box; it’s smoother and quicker than Microsoft’s desktop search, also used by Outlook in the latest version. Under the covers lies my favourite desktop database engine: sqlite. I’ve turned off the official Outlook search; anything to speed performance.
Xobni is free right now (it is a beta), so what’s the business model? Still up in the air, apparently. However, given the number of Outlook users, I expect it will be possible to monetize it. Apparently Microsoft tried to buy the company and was refused.