The biggest concern I have with services like Foursquare, Gowalla and Twitter isn't whether they have value (I think they've clearly demonstrated they do), but it's whether or not they are building sustainable businesses. All these services seem like they are developing feature sets that their larger competitors (read Facebook and Yelp) will simply be able to add to their existing service as a major enhancement. Why would I use a dedicated service, when the broader services I do mostly the same thing?
TechCrunch: Facebook QR codes and location
TechCrunch: The Location War