PIMs suck big time. Even desktop ones suck. Outlook sucks. Everything else sucks because you cannot implement a feature which whill be lost when you sync to Outlook/Exchange. The main reason PIMs suck is that they are application-centric. There is "calendar" app, "address book" app, whatever else. They are very loosely connected, thus reasonable usage patterns cannot be implemented properly. It is all about object data backends and proper presentation. Proper PIM is "outliner" where you can link tasks, people, appointments, documents to some structure which is to be visualized with presentation modules which are seen as "address book" or "calendar" or "nearby map" or "todo list" or whatever.