I went a year or so back, took my Giant XTC, reasonably fit from commuting to work but was a bit rusty off road as hadn't ridden for a while.
Did it in about 3 hours, but I met up with some local riders and they took me off the beaten track, plus we stopped to chat here and there.
Climbs on the Monkey were a challenge but never felt too much. Nothing on the trail gave me the 'oh my god they're trying to kill me' feeling, but at the same time I found it a resounding challenge and had to concentrate on my riding. Nevel felt underbiked, even with the relativly XC steep geometry of my Giant.
Some of the Monkey descents were hard work due to braking bumps. Optional black sections if you're so inclined, I wimped out on a couple but as I passed them they actually didn't look that bad.
All in all I had a great time and will return when I can.