I started boating open boats in flat blackwater rivers. Then when I starting whitewater, I switched to kayak. But the first WW boat I wanted was an Atom --I just couldn't find one. When I finally got one, I loved it. After years of being in a kayaks, the transition was challenging but not too hard. The payoff was great.
The length of the Atom gives it plenty of hullspeed, and it is a very responsive boat. I think it turns as easily as my Jive, but it also resurfaces a little better (the paddler sits on a pedestal in the center of the boat so there is less weight over the bow).
I give the Atom a 9 for downriver travel. A 6 for small wave or large and shall wave surfing, and a 3 for steep wave surfing (it's bow wants to pearl). It's GREAT for class III and IV water. I wouldn't take it creeking or on big drops.
The outfitting can be painful on the knees, but it's not for girly men. Tough it out, quit your whining. Eventually you get used to it. Over all, I'd give it about 7.5 as a general grade (considering some might want a boat they can play in).
As a pure river runner, I give it a 9.
For beauty on the water, I also give it a 9: they are pretty boats.