The landings are AWESOME! But then again, I am coming from an 04 substance when I say that. First time I cased the wake though, it felt like landing in the flats on my substance. So no, I wouldn't say it's harder on the knees.
I just went from a 06 Watson with 06 Watson bindings to the new 08 Watson with 08 closed toe bindings and it has the same feel, but I think the 08 is snappier and feels a little lighter. This could have more to do with the bindings though. I think the closed toe bindings make it feel a little different (in a good way) but it's hard to explain. The landings didn't feel any harder to me. I'd actually say the Watson is one of the best landing boards I have been on as far as the impact on landings is concerned.