I guess dropbox works for most people because it's so easy to use and just fixes their specific problems (of keeping files in sync, quickly sharing them via url, etc).

Me, personally, I use Google Apps for emails and documents and so, naturally, I try to do as much as I can with it. Today i collaboratively worked on wireframes through it even though mockflow or mockingbird are much better at that. But they cost a bit more than I can affort right now. Also, it's nice to have everything in one place.