I think this is the best down river boat available, and is my favorite paddle boat on the market. Period. All the strengths of a canoe and none of the weaknesses. Great speed, good responsiveness in current when the boat is properly balanced with gear, and for the aesthetics crowd, it's lines are beautiful.
Among the nice features are lots of available accessory gadgets for gear-junkies (if you go barefoot in your boat, BUY the Wussy Foot pads - they're worth it), the comfort of the seats and the obvious extra seat that's removable (seats and footpads are adjustable such that the boat can be converted into a 1-person dream cruiser), and the fact that this is the best boat for standing up in both while casting and while paddling.
Down river paddling will always benefit from being able to stand up periodically to scan riffles ahead, and this boat facilitates that on Class I Rivers, perhaps on Class II for practiced paddlers. If after an hour of trying you still have difficulty standing up in this boat solo, it's because of your balance and not the boat's construct (at 6' and ~160lbs I have a high center of gravity, so no excuses). I love to use this as a standup down river boat on relaxed watercourses with Werner's standup Advantage paddle. The tunnel hull lets you learn how to fall forward onto your knees into the boat when you hit an obstruction so long as it's not broadside, so standing in this boat is far less likely to result in a swim than doing the same in a canoe.
There are three reasons I won't give this boat five stars.
1: It should be available in Royalex - the rotomolded plastic that it's currently made with is likely fine for inshore fishing, but down river paddling lets gravel and rocks carve up the hull. Made out of the former material, this boat would have better bounce, more memory (these boats will deform if left atop a hot car for several days), and longer lifespan. That said, I've paddled watercourses that pounded the rotomolded boats and they do hold up well - just don't faint over the hull scars.
2: Despite ACK's current photo (11/2010), the toggle handles (which I prefer) are no longer a feature of this boat. For those of you who prefer overmolded handles (the soft rubber loops bolted at either end), that's a good thing since 2011 models have these. NWC claims this change came about to make the handles quiet when paddling. Since the toggles were bungeed (the only kayaks I know of that did this) there was no noise in the first place. However, the bungee cords likely weathered out on earlier models and were difficult to replace.
3: These boats are super fast, but are subject to windage. If you will be using the boat on lakes and inshore, pay ACK to install the rudder or plan on rigging one yourself.
For folks who like the feel of a reclined, two-blade paddling experience found only in kayaks but also like an open top without the canoe wobble, this is THE boat. Worth every penny, and it's one of my hardest opinions that you won't find a better place from which to purchase this boat than ACK.