If you're on a search for usenet nzb file downloaders for OSX I would really recommend you to take a look at the special OSX Version of Sabnzbd over at br41n.com

It's packaged in an .app file and simply works pretty fine :)Simply download it, start it, go over to http://www.localhost.com:8080/sabnzbd/ and start configuration.

It does the whole unrar, par2repair stuff and seems to be in active development

Been there, took some pictures… can't belive it took them so long to build a bunch of empty halls