It's not flat and not mountainous - some rollers, some false flats, not really very much actual flat. My altimeter said 5600 feet of climbing (in 92 miles of it, my computer shut off for a little while), lots of rollers, one steep climb (unfortunately, on a rather heavily trafficed road) - it's rather scenic, actually, in a pleasant non-spectacular sort of way. Nice views of Pikes Peak. But not like the P2P or anything.
It's very very well supported - tons of rest stops. Definitely a good chatty ride.
The biggest downside of this ride is that it's HUGE. Many cyclists of with a variety of group riding skills, including none at all. If you're up for that, it's pretty fun.