I started using the Davy and double Davy several years ago because of the ease in tying it. This interesting video covers several different terminal knots and their relative strength. It's nice to know that the Double Davy is not only easy to tie but also one of the most effective. I still use Duncan Loops when I want a free swinging fly but for my nymphing this is pretty much what I use now.
http://www.youtube.com/watch?v=uBnuYu9P6yU