I will recommend Adventure Cycling maps, as long as the route goes to where you want it to go. As for benefits, check out these pages over on their website, where they list them in detail:
http://adventurecycling.org/routes/maps.cfm
http://adventurecycling.org/routes/mapdetail.cfm
Here's an image of a map panel, detailing the features.


As for overcrowding, depends on what your definition is. There's a likelihood you'll see more touring cyclists on an ACA route than touring off the route. If you're the type that never wants to see another cyclotourist while you're on a bike tour, then it might be wise to stay away from their routes. I've ridden various sections of ACA routes, and have seen anywhere from none to a dozen or two cyclotourists a day. It all depends on what route you take, when you take it, and which direction you ride.

For example, I've seen the most cyclists (a dozen or two a day) on the Pacific Coast Route, but that has more to do with it being the Pacific Coast, not because it's an ACA route. I ran into a couple to as many as ten in one day riding the TransAm through Oregon in early June. I've encountered two to four cyclotourists a day on the Sierra Cascades route around the Columbia Gorge. And crossing parts of the Northern Tier last June, I didn't run into any other cyclotourists, but when I talked to locals they said they've encountered other tourers recently.

To me, the biggest con to ACA maps besides expense is that they give little to no information of anything beyond the mapped route. If you want to take side trips or deviate from the route, you better have some maps or GPS.