I personally like the DDNN over the traditional b/c I feel like I get more of a break away from work. You get a partial day off in between the switch from days to nights, so it is not that bad working 4 shifts in a row.
It is also much easier to take vacation with DDNN, just take your set off and you have 2 weeks off
Which is great for the people who like to travel. You usually have a whole new set of patients when you come back after your set. One of the down sides are: more nights, some months you are working more weekends, you are usually a bit more cranky on your last night, and really it feels like you get only 4 days off because you are sleeping most of the day on your first day off.
I guess there are pros and cons to both, but I personally still like DDNN over a traditional rotation. I don't mind my job, but I really do need that break from work to keep coming back with a smile.
(I wish we could work 3 12s a week and this would be considered full-time hours. They seem to get this in the states, to me that would be the best schedule)