Brightkite as a social networking service excites me and I was thrilled when they came out with an application for the iPhone. The iPhone app offers all the flexibility and functionality found when accessing Brightkite via the web with the added benefit of using the location ability of the phone to more easily mark your position.  

Users have the ability to checkin at a known location, allow yourself to be located and then checked in, or search for a nearby location (based on the iPhone locating you) to check in.  You can also check in an leave a note or check in and post a photo. 

Once checked into a location one of your options is to choose the “nearby” button and you will be presented with a list of other people that have checked in to that location or a nearby location.  From there you can explore the locations where they checked in, make new friends, or leave comments for those that follow to find.

The only thing that I have found that you cannot access from the app is some of the fine grained settings, but they rarely are accessed save when setting up your account.  

I highly recommend this app and Brightkite too.  If you need an invite hit me up @gmail or DM me on Twitter.