I would suppose that the routes it would make more sense on are the ultra long haul ones. On these weight is a huge factor and therefore cargo capacity limited, but you are carrying around a bunch of unused volume.
I could see a 3 class configuration, with some sort of suites for first class, a business class product much like today with nice food and bar etc. and economy class with bed access sold separately for say $500-$1000. You'd have to be careful not to make it too attractive or risk robbing sales from business.
Essentially the bed class would be the replacement for premium economy. You would not have a PE seat.
The airline would also be able to flex up and down the "PE/bed class" seats just by loading another pod of beds if sales in PE were looking good.
I would say that any route under 8 hours (ideally 12+) would not make sense, due to general faffing about, and the need for 5+hours in the bed to be actually useful.