skinned and snowboarded it in jan or feb. i'd highly recommend that, though the only real snowboard worthy part was the summit cone. zero viz when i was there with horizontal snow and maybe 60mph gusts-an estimate from one of the locals i skinned up with-couldn't qualify that for sure, but there were times the snowboard acted as a sail when i put it together and threatened to take me for a little ride.
as for climbing it in sept, since marcy is the most popular climb in the daks, i think you'd have to expect a herdpath of people and a pretty crowded summit. if you are planning on camping out you will need to take a tent, don't count on getting a lean-to on a weekend anywhere around marcy dam or avalanche pass. also, though the black flies are not as bad this year, you would serve yourself to come prepared for them - i.e. wear breathable long sleeve tops and pants and pack a bugnetting hat.
it is not a difficult route by any technical means, but it is a long day...should be around anywhere from 12-17mi. roundtrip depending on where you start. enjoy-it is certainly not to be missed