I have used the Dual Drive extensively on a recumbent. If you forget and stop in a high gear on an ordinary derailleur bike, you can just put a foot down, lift the bike and twirl the pedals while shifting down. On a recumbent that is well nigh impossible. Because the DD hub gears can be changed without pedalling, you can use the hub to select a lower gear and make your escape. Apart from that, the rest of my remarks will have nothing to do with recumbents.
Good points of DD:
Front and rear shifters are both on the same unit on one side of the handlebar.
No need for front derailleur mechanism, just a single chainwheel up front.
Hub gears protected from weather and muck.
Adjustment of hub gear important, but straightforward.
Less good points:
You need to get your head round the need to keep pedalling when shifting the derailleur part and to stop pedalling, briefly, when shifting the hub gears.
When dropping the back wheel to fix a puncture, the hub gear cable has to be disconnected. Getting it off and back on again can be a bit of a pig.
Hub gears can be a bit delicate and are difficult to fix if they go wrong.
If you need really low gearing avoid DD as the hub has a limit on the amount of torque it is able to take. Put too small a chainring on the front and you will exceed that limit, and may damage the hub when stamping hard on the pedals. Strong heavy riders may overload the hub, giving rise to some of the bad reviews.
I have ridden my DD for a good few hundred miles in all weathers. It has been entirely reliable. The different shifting techniques soon become second nature.
If mine finally dies, I am not sure if I will replace it though. For one thing, I am not sure if it is being made any more. I might go for a wide range hub gear instead.