It's much more useful in a dense urban area. You can e.g. see that your friend is on the same event as you. Or in a caffee two blocks from you. Or in a cinema watching a movie, so you now have a conversation topic.

One of my experiences of using Foursquare is when a friend of mine checked in to a particular takeout food venue we fancy. A notification came, and a guy sitting next to me got his phone and call the mentioned friend to also buy food for all of us.

Such random situations were the main value of Foursquare to me.