I have always just used the velcro top ftm pouch from The Transitional Male. that way I can wear whatever underwear I want and my business stays put.
In the interest of full disclosure I'm an underwear enthusiast/snob. I don't care for the idea of being limited to just the "packing" underwear currently available. Not to mention, If I'm going to pay that much per pair they had better be a lot sexier than plain black generic looking underwear.