I owned the 9.0 for several months this year and took it through its paces on the Potomac river, class 1-4. I am 6'4" at 185-190 lb and this boat fits me great, comfort-wise. It also floats me quite high (I am at the low end of the weight range). Being light in a big boat is great for staying above the water in rapids and surfing in holes. The 9.0 is this way more "retentive" (willing to stay in the hole) and continue to surf, compared to the smaller volume 8.5 model (the 8.5 gets flushed out much easier).
I found the 9.0 extremely easy to roll, not any harder than the 8.5 and certainly easier than my sea kayaks. Perhaps the other reviewer needs to modify his sea kayak based roll a bit for the way a white water boat rolls - the Axiom is amongst the easiest to roll.
The 9.0 catches and stays on river waves about as easy if not easier than the 8.5. It is a bit longer and a bit wider, so overall speed is probably similar, but the bigger volume means there is less draft and thus it spins and sides about more easily than the 8.5. The 8.5 is a lot more maneuverable, more nimble, more edgy. The 9.0 is more confidence-inspiring in "funny" water at the edge of eddies or side-surfing in a hole.
Both the 9.0 and the 8.5 I find easy to control on the wave and during cross-ferry. To me they are great for those who want to play the river but don't want to get a full-on playboat and want to have speed for attainments.
So a 10, as I have not found any particular issues with the boats for their intended use. Perhaps, I would prefer a slightly higher and closer together knees position, but some like it as it is - that's a personal thing - so I won't ding the rating for this.
To me the 9.0 (for my weight) behaves like a solid high-volume river runner. The 8.5 is a fun and more playful and agile river runner. I had both the 8.5 and 9.0 concurrently for several months in order to pick one. Sold the 9.0 because I mainly play on my local run. If I was mostly doing downriver runs, I would have kept the 9.0 instead. So either might be the "right" boat - depends on the paddler weight, dimensions, and intended use.
The 9.0 and the 8.0, as any other surfing boat, completely change their character when they are on a smooth big wave. When they begin to plane and pop out, they fly - that's when one can use these edges to carve and do cutbacks on the wave face. On the Potomac at flood levels some really nice standing waves form that are 3-5 feet tall and offer enou speed for the Axioms to start planing a bit. Those are the most fun.
I would not recommend the 8.5 for anyone over 200lb though, even if they fit - the boat just rides too low and cannot offer some needed buoyancy to begin surfing or to do flat spins on slower waves for instance - it becomes too edgy. It would be good though for stern squirts on flat water and other tricks that require a smaller volume boat - the 9.0 I could not stern squirt for the life of me (too much volume) but I can the 8.5 and if I had another 20lb on me it would be even easier. So again, weight vs. volume for the intended use...
Finally, the 9.0 was well built and did not leak at all. The 8.5 with the same skirt leaks some through the cockpit, so their cockpits are not equally watertight and it is not just because the 9.0 rides higher and less water sloshes on top of it - even with rolling and surfing instill get less water in it than in the 8.5. So not sure what the issue is...
