Not sure about the profit part, but I do enjoy keeping a GPS log of my flights. Sometimes I share them with friends and family, but usually I just take a quick look when I get home and then save the files for posterity. It is a fun way to recall details of the trip you might otherwise forget - like deviations for sight-seeing or traffic. For example, recently I got checked out in my flying club's Piper Comanche.
Since this plane has a retractable gear, our insurance policy requires me to have at least 10 hours of dual instruction. To build time I planned a cross-country flight to Durango, CO, and got some hypoxia/oxygen training as well, since we flew at 11.5K MSL. Here's a map of the northbound leg of the trip:
I generated this map using MotionX GPS HD on my iPad WiFi, with the external Bad Elf GPS. There is also a version of MotionX for the iPod touch and iPhone, which I've used in the past. Thanks to background location service support in iOS, it's easy to start the track recorder in MotionX and then switch to ForeFlight, my primary aviation app. MotionX will keep running in the background, and even give you voice prompts every 10 minutes to let you know how far you have traveled.
Once I arrive at my destination, I switch back to MotionX and stop the track recorder. At that point, the GPS data is saved to memory and can be shared later when I get back home or within range of a Wi-Fi access point.
If you share your track via email (from within MotionX), you'll get a nice summary of the track, a link to the track overlaid on Google Maps, and two attachments in GSX and KML formats. I'm sure there are tons of logbook programs that can import these files, I just haven't had much time/desire to seek them out.
GPS datalogging can be useful for lots of scenarios - hiking, biking, running, boating, driving, etc. There are several good generic apps like MotionX and GPS Kit, or you can use activity-specific apps like RunKeeper, MapMyRun, etc. Most of these apps support exporting data to standard formats for sharing across sites or importing into programs like Google Earth. For photography buffs, there are programs that can automatically geo-code photographs by syncing up the timestamps in digital files and the GPS log, even if you didn't take the photos with your iOS device.
Personally, I enjoy checking out my pattern tracks. For example, here's my landing at Durango where I overshot the turn to final a bit. The second photo is from some pattern work I did a few months ago at DVT - you can see how wide the pattern becomes when there is lots of student traffic.
When I do finally get around to working on my IFR ticket, having the ability to review my flights from both bird's eye and VNAV perspectives will be very informative. As with anything, once you have the data, there are all sorts of things you can do with it!