Definitely they are hit and miss; but I have found that to be true in all forms of networking. This should certainly not be your only avenue for networking.
I take multiple approaches to networking. Meetups, events, conferences, calling/emailing people I already know, cold emailing companies directly if I know they use a technology I specialize in. I also wear T-shirts almost constantly that are simple and state the nature of my business and a phone number to reach me at.
Does the T-shirt approach work? I've wondered what would happen if I put my details on my laptop skin, as I work from a lot of SF/Valley coffee shops :)
I have bought large numbers of T-shirts and give them away as well as wearing them myself. I honestly don't have any statistical evidence that shows they have worked to gain me more business. However I always wear these when going somewhere that I specifically intend to network. In the same way as having business cards does; company branded T-shirts lend credibility to your business and I have been approached and directly asked questions about my business from wearing the shirts.
And believe me; I totally understand the uncomfortable at introducing yourself point. I am an introvert by nature. I have a rule for myself when I go to these places; I force myself to do a minimum of 2 new introductions each time. More is better, but a minimum of 2.