I've made the climb exactly twice, earning the "fool" award for myself. Although I made it to the top the first time without using oxygen, it's not a bad idea to bring a can or two to ward off altitude sickness, and IIRC 7-11 sells cans fairly cheap. They're very expensive as is everything else on Fuji, although if you have enough money to spend it can be easier to buy water, etc. on the way up instead of carrying it. jpatokal mentioned crowds, and I had to wait in line several times during my first climb - on a weekend in August, which is probably one of the worst times to go. Last time I went on a weekday and things were much easier.

Beware of climbing in October! Most of the shacks and lodges on the way up are closed, and emergency services are reduced. You're also more likely to encounter bad weather.

Also, many people think going back down is worse than the climb. If I make the climb again, I'm definitely bringing hiking poles! You really need support on the descent if you don't want to fall down about 50 times