Combining e-mail and calendaring, and integrating it with the rest of the Office Suite was a masterstroke on Microsoft's part. It has streamlined office communication to the point where even shops where development is done on Linux machines have Exchange servers handling company email and I get funny looks from people who ask me, "That meeting didn't show up on your Outlook?"

Outlook is broken only if you give a shit about openness or standards. Most people don't. Most people want to solve the problems immediately before them to make their lives easier. This Outlook achieves with aplomb.