The route, scenery and overall experience were GREAT. If you're thinking of taking the trip, then you definitely should do it.
It is very satisfying to have a route mapped ahead of you, and all the freedom in the world to move forward along that path. Get up in the morning, ride through the landscape until you're tired, find a place to set up camp, eat some dinner, and go to sleep. Over and over again. No schedule, no obligations, no worries. This feeling doesn't develop on a one or two week trip. You need enough time to establish the rhythm and sense of freedom.
We were somewhat nervous in advance of the trip because neither one of us had any experience riding mountain bikes and we were worried about whether we would have the technical skills to pull it off. We were fine. There are some short rough stretches in Canada, a couple short rough stretches in Montana, and quite bit of rough riding in New Mexico. But those tough places early in the trip are short enough you can just walk your bike if need be. Starting on the south side of the summit of Indiana Pass in southern Colorado, the road quality was inconsistent, and sometimes included long stretches of poor road. But this didn't start until we'd had a lot of miles to practice, and by the time we got there we were able to cope quite easily.
The ACA maps were quite good, and we were able to navigate without using supplemental USFS or BLM maps. We will comment later on the very few places the maps were misleading. A GPS is not necessary at all. A small handlebar mounted compass is useful, but not critical. On the other hand, an accurate odometer is critical; be sure to bring instructions for how to adjust the wheel circumference setting in case your not synchronized with the ACA maps; make sure every member of the party has a working odometer; if you're traveling solo consider bringing a second computer because it's not all that uncommon for a computer to fail.
The scenery is lovely. However, it isn't as lovely as what you will get backpacking where you can get out into wilderness areas away from the cows. None of the mountains were as nice as the southern high Sierra Nevada (our stomping grounds), not by a long shot. So if you want beautiful mountain scenery, get out your hiking shoes and backpacks and head to the high Sierra (or the Wind River range in Wyoming, or Glacier National Park, or your favorite high mountains). New Mexico was lovely, but not nearly as lovely as being on foot down in the slot canyons of the Colorado Plateau. However, as a whole package the ride through all those diverse landscapes was a beautiful and wonderful experience.
The guide book was useful for giving general information about what to expect in each section. However, we consistently found that the campsites they recommended were not as nice as campsites we found on our own, and we didn't attempt to follow the daily itinerary of the book. In addition to having nicer campsites, we like the open-ended feeling of starting each day with no destination in mind. Finding nice places to camp was never a significant problem.
Adventure Cycling offers this trip as an unsupported group ride (trip leader + 14 participants) every other year, including 2008. If you're a social person, or if you are uncomfortable traveling alone and don't have a companion, then that's an option. Otherwise, go alone or go with another person so you have the freedom to set your own pace and agenda.
About those cows... This opens a rather large emotional/philosophical/religious can of worms. It seems that people have "messed with" every acre of land they can reach - put each acre to it's best economic use, be it mining, timber, housing, ski resort, golf course, strip mall, etc. If an acre doesn't offer any other significant economic value, then you can always put some cows out there and graze it. The ranchers we talked to were extremely nice people who are doing what everybody else does -- earning a living and raising their families. Each rancher and each cow does no more damage than the rest of us do when we go about our business of building a house and earning a living somehow. But grazing in dry climates alters the fragile top-soil and subsequently the entire eco-system. That's fine for any individual ranch. But when you see it on every single acre for a thousand miles in a row -- well it really hits home that we humans have left very few acres of land alone for nature to take her course. This was a very depressing part of the trip for us. The only significant grazing-free stretches were the parks in Canada, Teton/Yellowstone, a section in Gila National Forest in NM, and a section in Coronado National Forest in Arizona (off route). Those sections may have represented a total of perhaps 200 miles in our 3100 miles of riding - not much. Those were precious beautiful sections that we cherished.