I know I'm about two years late to this, but I'm really digging Foursquare. I remember meeting the guys behind Dodgeball in NYC before it was acquired by Google, and seeing how people used the service, but it seemed like an idea that could only fly in a densely populated place like Manhattan with its plethora of choices of things to do.
Fast forward a few years and I load up the foursquare client on my phone for the hell of it and notice that most of the downtown restaurants in my little neck of the woods an hour from Portland, Oregon are all listed with mayors, tips, and everything else. As I've driven around Portland and flown down to Southern California, I've been constantly surprised at the depth of coverage and wealth of tips available in larger cities.
The app is fun and using it is very much a game, but what I'm most interested in is how it's both a useful utility, but also has personality because it's basically crowd-sourced information from regular people. Similar services like Yelp feel more like a yellow pages with comments thrown in, where Foursquare is the exact opposite: it feels like a giant community blog with some maps thrown in.
As their database fills out, I'm interested in all the ways it could be harnessed for other applications. I could totally see a Fuelly/Foursquare mashup where you both check-in at a gas station you're fuelling up at and you can add your mileage stats to Fuelly at the same time.
Anyway, I know it's been around a while and is old news to every friend in San Francisco and NYC, but the app is useful even in the 'burbs and rural places now that more people are using it. I'd say it's officially hit mainstream.
Oh also: two quick tips. Connect your profile to Twitter and Facebook, but be sure to turn off automatic status update check-ins. Those are almost always annoying to your followers. Second tip: only add friends you've dined with in real life. The app automatically exposes your mobile phone number to your friends list which is a new level of intimacy for most social software. So unless you want to get rung up by random internet strangers from every other social network you're on, keep your friends list tighter than usual.