(My favourite kink-friendly topic!)
I have been practicing chastity and orgasm denial, on and off since January 2006. My longest period was quite recently; September 2009 to approx April 2010.
I have used the CB-2000, the CB-3000, the CB-6000S and the CB-6000.
My current chastity weapon of choice is the CB-6000. As long as you use the right size cuff ring, I find it extremely comfortable to wear 24/7, including sleeping and nocturnal erections.
As a starter model, I would recommend the CB-6000.