I know basically what APRS does.  My question is who uses it and what for?  I understand if you were working an event, it would be good to know where everyone was.  I know if you were on a trip, people could follow you.  I don't work events however.  I'm trying to convince myself I could have fun with it, but all I see is that people will be able to track my every move.  Help please.

I know a local ares group has used for event support, and commo ride alongs.
For a bike ride event, they had support vehicles with aprs beacons overlayed on a map in real time, and could tell at a glance where the first and last riders were, and where all of the sag support vehicles were.   This allowed them to make faster decisions when needing to send an asset somewhere, rather than asking who is the closest to a particular point, etc...   The commo ride along, hams accompanied sheriff deputies, in an excercise to simulate a failure of the public safety repeater system.   APRS allowed dispatch to know where all units were.  
RB

There is a group here that is into desert exploring and exploring abandoned mines. There is a good write-up on using it HERE.

A lot of storm spotters use it. I use it when I'm out chasing.



Some EMA's use it for locating search & rescue assets.



I keep mine turned off unless I want people to know my whereabouts....like when I'm storm spotting. It makes it easier for net control to look at a map online and see the location of all the spotter assets in the feild. Easier than trying to keep all the spotters locations straight in their head.



We also use it on mobilizations and convoys into a disaster area so that if the comms unit doesn't have access to weather radar, accurate info can be relayed from the net to the convoy.

There's more to it than just tracking, there is also messaging as well.  I'm still exploring it and its capabilities and thus far like it very much.  From a HAM point of view you can see those you are making contacts with if they are using APRS at the time of contact.  Also works as a saved tracks that you can access later on if you want (data goes back to 2009) to show some one where you were for a given event/activity (ie stalked and down a deer or elk).  Last black powder season I used GPS to find a radio I lost by retracing my tracks.  So I guess you are only limited by your imagination.  

Lots of small aircraft pilots are using it for tracking and emergency beacons.

http://www.vansairforce.com/community/forumdisplay.php?f=104

Lots of good info about installation and practical use.

I use mine for messaging and tracking. Also for weather at a remote site. I working with an emcomm group to add it to their capabilities to help track assets and use for simple messaging.