OK, I've had a lot of boats! Mostly I paddle a small Jackson, if I'm coaching or stepping it up I use a big boat. From 2010 until 2012 I used a Nomad 8.1 for this. I demoed a friend's Mamba and liked it. I suffered from terrible back ache in the Nomad and so decided to change.
I find it really hard to say anything bad about the new Mamba. The old style one was a bit catchy on the stern but the new one is super stable. I recently took it down the Upper Dart in low water and had it switching from edge to edge as I worked my way down the steep boulder fields. The boat is fast in edge transition and doesn't feel catchy at all. I got knocked off line and ran a section backwards, before the next rapid (right on the lip) I was able to turn the boat very quickly to get forward again and still get a boof in over the waiting hole.
It does boof well and like most folk I've occasionally messed up and landed on an edge. As long as you brace on that side you really don't have to do much the Mamba soon recovers its stability. I paddle the creek version and have had no back ache since swapping from the Nomad. Also the step out pillar has a curve towards the nose of the boat and this is perfect for hooking your hand around when carrying it on you shoulder.
So: In summary... Fast, tracks really well but is also great for dynamic 'slalom style' paddling, boofs well, super forgiving and comfy. That's a lot of ticks in the box. My roll isn't too bad so saying I find it easy to roll doesn't mean a lot. The one thing I'll add is they say it's a great solution to the one boat does it all crowd. I'd agree with this up to a point. It would be a real dog in ocean surf and let's face it nothing with that much volume is going to be very playful. That said it carves up clean standing waves and will spin if you really chuck your body into it. The best boat I've ever owned. I absolutely love it. Demo one if you're looking for a river runner
Dagger includes outfitting for leg and seat adjustment and the seat itself is very comfortable. The backband is adjustable and provides great support as I rotate my torso during paddling. A great resource are the online videos on the Dagger website that explain out to use the provided padding for outfitting my boat.
The 8.1 does fit me, but I am probably at the top end of height and leg length. I am 6'1 and 190 lbs. My paddling partner is a bit shorter, but heavier and fits the 8.6 Mamba better. I have yet to have the opportunity to paddle above a Class III, but the fall paddling season is here!