Here's my two cents worth on that:
1. Most book signings have turned out less successful than I'd hoped (see posts on My First Book Signing Fiasco or Eat More Chicken-Book Signing Fiasco #2).
2. The ones that were successful happened when I went to my hometown and people I personally knew came.
3. The ones that were least successful was when I set something up in a booth or table by myself in a non-bookselling context (like at a Chick-Fil-A or outside a library).
|Looks really impressive, but I ended up selling only 1 or 2 books. I did get free supper though! =)|
4. If you can be part of something that is already a selling context, that is better (like a town festival where lots of people have booths set up).
5. If you can sell books alongside some other activity, that is better (for me, like a human trafficking expo, or a speaking event on trafficking)
Basically, if your ultimate goal is to sell books, regular sit-at-a-table book signings are not worth it, in my opinion (unless you have a New York Times bestseller that everybody already wants, or you're really good at selling things to people). If, however, you want to get your name out there and gain exposure for your book, book signings are worth it because people get to know your face and your book, and the more familiar they feel about it and you, the more they may talk about it with their friends or buy it next time, or offer to have you come speak, etc.
In conclusion, I haven't found book signings to be great for actually selling a lot of books, but I have found them to be worthwhile for the connections and exposure they bring.
You can decide yourself if that's worth it to you.
P.S. If you come up with some amazing, foolproof method for making books signings a success, do let me know!