I remember at one point encountering the Open Borders site, considering that it sounded like a pretty good idea, then reading through much of the site and becoming decreasingly convinced as I read the specific arguments, which consisted of more holes than solid points. Recently, it's come up again, specifically in an interview with Caplan which was going around (I saw it via Kaj). Again, I was initially intrigued by the idea, but the more I saw of the actual arguments, the weaker they seemed. He seems to routinely deflect the significant concerns with non-denials and never actually address the pragmatic reasoning against it.