I posted above why I like Org-Mode. Here are a few things it doesn't do well.

1. Because it is text-based, its visual summaries are limited. Like, there's a calendar, and for org-habits a simple color-coded chart to show consistency, but that's about it. If you're looking a week or a month ahead to visually see how many tasks you have coming up, the org-agenda is text-based.

2. While org-mode can sync with Trello, Beeminder, and a couple other task-management systems, its integrations are pretty limited. If you want to integrate with Contacts or whatever you'll have to work a bit.

3. org-mode's part of Emacs, so you'll need to be comfortable with Emacs.

Contrast this with Sunrise Calendar. Sunrise Calendar grabs gmail messages and appointments, iCal, google calendar, and displays them all nicely.

Pluses and minuses, depends on your needs and what you like.