If you are flying in and out of Tokyo Niseko will probably be too far to travel. Try Hakuba instead, I went in Feb 2011 and had a great time. Hakuba doesnt have the kudos of Niseko, but on the plus side the conditions are usually much brighther in comparison. Powder is less frequent but they still get their fair share of it and when it does dump it doesnt get tracked out as quickly.
Its pretty easy to get to from Tokyo. Head over to Nagano on the bullet train and then catch a bus from Nagano station to Hakuba. I stayed with Hakuba House, run by an Aussie guy called Mick and his wife Sarah, I believe it has changed hands now but I should imagine it is still just as good.
Rob