I'm a dual iPad owner with a Mini 4, previously 9.7" models, and currently 12.9". The mini to me is the ultimate consumption device. It has the best PPI of all of the iPads. It's great for reading books or Internet, time waster games, even short videos. I also use mine as a remote for iTunes, my AV receivers, TV with the respective manufacturer's apps. It's a perfect device to keep docked in the family room.
OTOH I enjoy the bigger screen iPads for writing/note taking, editing PDFs, brainstorming -- things you might do on a regular size sheet of paper or computer screen. I don't really see the mini as a competitor to the other iPads. It's the eyeglass screwdriver kit whereas the bigger iPads are regular size screwdrivers.