I don't have facebook or foursquare so am unable to try it. But from the pitch I think it might be useful. I worry though about anything that encourages the herd mentality. I like to go to places I know to be fun or interesting rather than just because there are other people there. The serendipity angle to life is oft overlooked. By herding its hard to discover anything novel.
But I will try it when you have gmail log in.