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.