I'm not a big fan of location based networks like Foursquare and Gowilla. So far I havent seen the value for me to check in and out of places (yet). One area of these networks that I do think can be valuable is the aggregation of such data and begin to visualise and draw conclusion. I visual digital diary in other words.
Blue circles show where you have visited. Places you have visited are represented with larger circles. A time series chart on the bottom shows your relative volume of check-ins over the days. Let the animation play, and a yellow line shows up connecting your current location to the previous. So what you get is something that sort of shows your movements.
Shaded regions on the map also become more opaque as you visit them. You can see where you've been, and more importantly, places you still haven't checked out. This feature only seems to be available for certain regions though.
Since you likely don't check-in everywhere, it's going to be missing some things (especially if you're not very active), but for others, you should find it interesting.
Souce: Flowing Data