Johan,
I am going to go in a different direction than the general consensus and recommend the SkiErg. My gym recently added one into the mix, and it is fantastic. I cannot envision any exercise/tool/machine that can provide the same endurance building opportunity. Pullups, dips, cable work, all great for strength, but you cant get that long slow grind. As another responder posted, it is a different movement, more ski specific, but its not that different. Having said all that, I am in the poles on the way down camp. Have found using them on the way up increases my speed but also increases my fatigue. Is there a market for resale if it doesnt workout?