I’d say Xobni is typically great for people that really don’t need a fullsize contact manager with opportunity tracking. Xobni is much less formal and requires less of a commitment – but it obviously also offers less functionality than a full-blown contact manager does. It doesn’t store any contact information for you, it simply makes it much easier to work with the information that’s buried in your inbox somewhere. In that sense, it’s more of a plugin (albeit a very cool one) than a complete application.