I'm in the informed position of being able to comment from extended experience of using a set of carbon rimmed wheels.
I own a set of Easton Haven Carbons and have had them for a little over eight months.
You need to split the evaluation process down to two basic questions:
First - is there an inherent difference in the way a carbon rimmed wheel 'feels'?
Second - what is the weight advantage offered and what benefit does that confer on the way the bike rides?
First question first - do they feel different?
My experience is yes they do. To what degree they feel different depends on how sensitive you are, whether you're just coming to them having never ridden them before, how you ride etc.
Almost everyone who has tried my bike comments on the 'feel' of the wheels; these comments tend to focus on how stiff they are in turns but how compliant they are over rough ground. The stiffness is also noticeable in terms of steering accuracy and tracking; the bike feels a little more composed and has more 'integrity'. It's the same kind of feeling going from a very light, relatively flexy frame, to a bullet stiff carbon frame. There is a subtle but to a lot of people, very noticeable difference in 'feel'.
Not everyone who has tried my bike says they can feel a difference but not everyone is the same so hey ho. Everyone's experience is right as far as they experience it.
Second question - how much lighter are they and what benefit does this have?
The Easton wheels are 1450g and have the no questions asked 2 year guarantee (which I can vouch for being honourable - more on that in a moment). At my weight - 107kg at the moment - and riding style I couldn't possibly get away with a wheel set within even 300g of that weight. The lightest I could go to would be a Crossmax SX. So these wheels save me not far off 1lb in rotating weight.
That is a huge difference and extremely noticeable, especially on a bike that weighs 31lbs without the carbon wheels. It makes a very big difference in the acceleration, turning and the overall feel of the bike. And I love it.
So the performance advantage is mostly in the fact that you can have a set of wheels that are extremely light but also incredibly stiff. The weight is a bigger factor than the stiffness I think, but having owned a very light but extremely flexible wheelset (an original set of Rovals that weighed about 1550g) the combination of light and stiff is what's most appealing.
Here's the rub though; if you're going to go the Edge AM/XC rim route, you're not going to get a wheel that is much lighter than say a Crossmax SX or ST equivalent. The Edge rim is 410g (give or take 10g I think), so a complete wheelset, with 32 spokes, is still going to only 150g lighter than say an equivalent build Stans Flow rim. It will be a lot stiffer though and have far greater longevity.
Importantly, the reason the wheel can be built so stiff is because you can run much higher spoke tension than normal, without any risk of cracking the rim. That stiffness is key so you need someone who knows what they're doing with the wheel build and has experience of working with the Edge rims. Perhaps North West Mountainbikes would be the place to go?
I came very close to opting for the Edge wheels on account of I already had a pair of Hadley hubs to build them on.
The reason I went with the Eastons in the end is because of the warranty they offered and because I couldn't get the Edge rims for 2 months.
Now the warranty; I have used this twice. I took the wheels on my AM bike to Verbier in August. I was not riding the DH tracks, but the tracks were very rocky and with my weight and not being too slow over rough ground, the rear wheel didn't last more than three hours before I blew out two spokes. Once one spoke has gone, the high tension means the others are taking a massive load and they won't last long.
It was very disappointing and I am still not convinced that the rear is really up to the job but I need to get to somewhere really rocky again before I can test that theory - the Peak District is on the cards for March.
The wheel was completely rebuilt under the warranty and the bearings have also been replaced twice now. There is a known problem with the hub design that is being worked on by Easton and there will be a retro fittable update available soon.
So if I had the decision over again would I still buy them?
That's a tough one because the light weight really is incredible and really does bring the bike alive. Having had them now for a while, I can no longer 'feel' the carbon effect. They just feel like a normal set of wheels. However, I imagine that going back to regular allow rimmed wheels would highlight again the difference.
I am disappointed that I don't have a wheelset I feel I can rely on 100%. Making the choice again I would most likely go for the Edge/Hadley/CK hub option as that gives greater peace of mind and I know that no matter where I am, I could get a spoke replaced easily; also I know that the rear wheel would be up to the job.
So I hope that helps. Long post but maybe will give you some insight from someone who actually does own a set!