I find the MBTI incredibly useful. When I meet someone I immediately figure out what their MBTI type is. That saves me enormous amounts of time later on---I don't have to pay close attention to what they say or do. I just rely on their MBTI type. Sometimes I don't even bother listening to them or asking their opinion, which lets me use the time to think about other things, like HN. I used to use their astrological sign, but that required asking for their birthday, which was awkward, especially when I didn't get them a present.