I also recommend going with an individual guide rather than a bigger tour group. That way you have more flexibility to do and see what you want.
I would discourage you from going during any holiday period, particularly the upcoming National Day holiday. It will be packed like you can't imagine. A few years ago, I went to Huangshan during a holiday. The main (central) trails on mountains were jam packed to the point where you could hardly move. There were tons of tour groups and guides, all of whom insisted on bellowing through portable megaphones. With so many people there at one time, it was actually dangerous in places. So definitely go during off-peak times.
For those of you who are into hiking, here's a map.
http://www.colourhs.com/a/huangshantutechan/huangshanlvyougonglue/2012/0502/4504.html
The main access to the mountains is via a cable car (circle at the bottom). Bright Top peak is kinda the central scenic area of the mountains, and people go there every morning to watch the sun rise. If the weather is right, it's one of the places where you're literally on top of a sea of clouds. Quite near there is a bunch of short walking trails and hotels. I recommend that people stay on the mountain for a night, even though the hotels tend to be more expensive. On the mountains, signage everywhere usually includes English. Personally speaking, I think you need to spend a minimum of two days actually in the mountains themselves.
On this map, I've marked the western route (often referred to as the Western Steps) in red. If you're fit and have time, it's a spectacular walk and will show you the very best of Huangshan scenery. It takes a good 6 to 8 hours and involves pretty strenuous climbing and descending. Set out early in the morning. There aren't many people along this route and you can find some tranquility.
Like I said before, if you're going to the mountains, also make time to visit Hong Cun and Xidi villages, which are located in the counties surrounding Huangshan mountains. If you only have time to see one, make it Hong Cun. Also recommended is Tunxi old street and Jade Valley.
Generally speaking, 4-5 days is enough to explore Huangshan mountains and surrounding counties. Anything less is really not enough in my opinion.
