Best thing ever, the Mega qualifier run was probably the best ride I have ever been on, it is impossible to describe, it can only be experienced.
To start with, ignore those who say the track is not difficult -- there is some serious technical riding involved; I would say if you can ride the Fort Bill red downhill run comfortably on the bike you are planning to use, then go for it (it's not in the same league as the Fort Bill world cup track, but it is harder than any UK trail center black run I have been on).
Dual ply tyres and a chain device (dual ring) are a must (the course is littered by people who ride single ply tyres fixing punctures and folk without chain devices, like Tracey Mosley, breaking their drive trains). I would also recommend full body armour, mine has earned its keep.
I personally would not choose to do it on 140mm travel bike, I was glad for being on a bullit with 180 fork front. That is not to say it can't be done (plenty of folk do it), or that it will not be enough fun (everyone I met there was having immense fun, regardless of bike), but it is very punishing both on the rider and the bike.
The scale is impossible to imagine until you do it ... start training early, ability to hold onto the bars for 1.5h downhill is critical.
Oh, and the race is nothing like just riding the track -- in the training you look at what lines to ride and what not, but just about the one thing you can be sure of is that in the race is you will not get to ride your chosen lines at any of the most difficult sections.
But yeah, an awesome event ...