I demoed a Dynamic Duo last summer and took each of my kids (ages 4-11) down a short class II/III run one at a time. I absolutely fell in love with the boat and the idea that I could take my kids down harder (more fun) runs than they'd be able to handle in their own kayaks. I also liked the idea of helping them learn various techniques, possibly more quickly than they would in their own kayaks.
I picked up a used Duo this spring and have spent more time in it than I have in my own kayak. It's been one of the best kayak related investments I've made.
Rolling with another adult who doesn't know how to roll can be tough. Rollins with my kids has worked out well - my oldest boy can help roll, my youngest boy just tucks and lets me roll.
I've only paddled it with kids on the rivers. The smaller the kids are the easier it is to paddle - they tend to be and stay more centered. The older/bigger they are the more their actions will affect you. In one rapid we caught a big cross current - I thought I had braced out of it - only to flip immediately after I finished the brace and relaxed. A friend who saw it all go down said that even though I braced, my oldest boy had his body/head out over the water and that took us over.
It's a long boat and has great speed but it can also feel like a tank to turn it. I've run Upper Mish down on the Poudre as low as 1 foot and it takes a ton of work to navigate the low rocky run. It's almost like rafting - if you can't go around it, go over or through it. I often have to tell my kids to stop paddling so I have time to make the move without blowing by it. (note - I'm used to uber slow river play kayaks). An upside to the long boat is you can more easily punch holes that might otherwise trip you up in a short boat.
My brother has an old TopoDuo and has paddled my Duo. His feedback is the Dynamic Duo is more ergonomically comfortable than the old Topo and overall easier to paddle, more forgiving, etc. I'm 6'3" 250+ and I can fit in the front and back cockpits. The back cockpit is huge and has tons of room. Some of the Duos have the Jackson bulkhead front and back (super easy adjustment, esp with kids). Others have footpegs front and back and sell (or sold) for up to a couple hundred less at retail price.
The boat is huge (140+ gallons) and heavy. JK lists it as 80lbs dry. It's possible for one person to heft it to the put in, throw it up on a car/SUV rack and take it off but it's quite the feat.
All in all I couldn't be happier with the boat. I've taken each of my boys out multiple times in it this year, as well as several of their cousins. Kudos to JK for re-inventing the Duo.