What I really like since my network of friends spans the world, were I to travel to another city, will find them instantly and discover spots off the beaten touristic paths, with my mobile.
For example, while I checked-in at the Apple Store, someone (using comments) asked that I look for a particular accessory he needed. Was able to render this service on the spot, commented back – and avoided him making the trip for nothing.
I find this pretty cool.
Another example: while checked-in at Indigo (bookstore) I stated what book I was reading. Then two people asked about it and why it was so important.
I find this empowering.
Why not try it? Join me on Foursquare and / or Gowalla.