I'm not a power user, but use Org every day. My init.el even drops me into my org todo list. Top uses:
Engineering notebook. Key combo calls org-journal-entry which I found on the web. It brings up a time-stamped narrowed buffer. I use this in every meeting where note-taking occurs. TODOs can be recorded directly in the journal and can be merged with other TODOs in agenda mode.
Linking. C-c C-l creates a nice looking hyperlink. I link to URLs, local files of all kinds (office, pdf, jpg, etc), or org tags. E.g. I keep a list of restaurants I want to try. I tag by cuisine type, and link to the restaurant's web page. Cycling to DONE time-stamps the entry so I know when I visited. I keep track of books-to-read in a similar way.
Other TODOs. I set the mode cycling to TODO -> IN PROGRESS -> WAITING -> DEFERRED -> CANCELLED -> DONE so I can handle a wide varieties of task outcomes. Also timestamped.
Spreadsheets. Mostly when I have a bunch of data already in a buffer and want to do some quick calculations. Quicker than firing up a real spreadsheet, though it's pretty limited.
HTML generation. Making pretty outlines/notes from my org files.
Gnu-pg. Seamless encrypt/decrypt of sensitive data.
Stuff I tried but mostly stopped using:
Org-babel. It was neat to run Clojure, Python, Ruby, and shell all from one file, pipe data between them, and make pretty tex output from it all. But day to day I rarely need that sort of power.
Calendar. I tried keeping all my recurring and one-time appointments in org. Problem is, I'm forced by work to live in Outlook, so keeping 2 calendars is not worth it.
Agenda. I used to view my agenda to merge everything in todos and calendar. I tend not to use agenda much any more.
TBH, I don't really have the discipline to rigorously update my TODOs all the time. Even so, I'd feel a little lost with out org. As with any todo system it declutters your head. When I realize there's something I should do, I need to get it out of my head into org, else I feel anxious that I'll forget it. Hardly matters that I only wind up doing 1/3 of them. I also rely on it for backup and sync of important notes, todos, etc. (via dropbox).
You'll get out of it what you put into it, but even with a few core uses (for me, notes, todos, and linking) it can quickly become indispensable.