We were there two years ago. It was great. We went for our Granddaughter's high school graduation at Misawa Air Base, and spent 2 days at Tokyo Disney as we headed home.
Magic Kingdom was great but more crowded than WDW a Christmas ( and it was an average day, no special things at all). DisneySea is like nothing you have seen. It was awesome! And significanty less crowded than MK.
Some thoughts to consider
We like personal space around us. Japanese don't need it, making it feel very, very close.
Long lines are nothing for them. We waited over 2 hours for Splash Mountain.
The staff is even more disciplined than they are in the US. Makes it fun to watch.
Eblong - Thanks. Japan is 9 hours away from us; rather than 14 to DLR and 20 to WDW. I've been thinking about it...just that the earthquake and tsunami put pause on the plans. If you notice the lack of personal space, then we're probably going to even feel it much more. Aussies are used to a heap more personal space and I feel the lack of it even in the US!