I'm honestly surprised by all the fuss over xobni. I have it running on my Outlook at work, where I get a ton of e-mails per day, and I frankly haven't found it to be very useful as of yet. Maybe I haven't fooled with it enough.
Search-done-right and the list of attachments sent between myself and a contact have removed a perpetual source of frustration in my workday. Maybe your situation is different.
I used to regularly run 5-minute long searches in my inbox to find a particular attachment that I needed. It was hell compared to, say, GMail. I haven't tried Xobni, but the search and attachment features are the right ones to tackle.
I heard that Outlook 2007's search is much faster than previous versions. Is that true? Maybe it isn't as fast as Xobni, Lookout, or Google Desktop Search, but people have said that it is fast enough.
I'm with you - I tried it and didn't find it useful at all.
To be fair: about a week after my initial comment, I've adjusted to xobni and I'm pretty sure it has saved me some time in sifting through e-mails.