Well, obviously it's a subjective opinion. I'll just say that my idea of good collaborative software is something a bit different than a pile of word docs hooked up to a web front-end and some other pages editable through a wysiwyg control that makes Frontpage 2000 look like a champ. But maybe that's just me. Obviously a lot of people find it useful. But to me I think it succeeds more due to the fact that it's an MS product and the extreme lack of robust competition in that space than it's due to any substantive positive qualities of sharepoint itself.

Edit: fixed wysiwyg typo.