I have fallen into a tree well - head first - and it was terrifying. It happened in my first year riding (about 12 years ago) at Shames Mountain in NW BC; old growth coastal cedar, hemlock and balsam trees in an area which gets 40 feet of snow a year, a pretty lethal combination for a noob who, stupidly, was riding alone at the time.
I saw the well at the last minute, tried to turn and somehow slid into it head first and came to rest pretty well in a vertical position with my board stuck fast across one side of the top of the well. Fortunately, the walls didn't cave in around me so I was not faced with the immediate liklihood of suffocation but I did knock down a fair amount of snow on myself in my struggle to get out. Needless to say I was scared shitless.
It was SO much work to get up into a position to release my bindings (the only time I can say thank god for step in bindings) but at least the well was wide enough for me to lift my torso up to reach my board. I had to make several attempts before I was successful and had to really focus on not panicking. To cap things off, I was on a flattish pitch so I had to paddle board out to an area where I could get back on my feet. Took well over an hour to get back to somewhere I could get back on my board and I was totally exhausted.
It certainly could have been much worse (I don't know what I would have done if there had been branches or the snow had immediately caved in on me) and it taught me enough of a lesson that I have never repeated the experience.
Chronicles of Gnarnia