I've seen the canyon from the rims, from the air, and from river level during a 6 day river run in July 2005. Running the river is by far the best way to experience the canyon, but most can't afford such a trip or that kind of time. Still, the solitude (read: no crowds), immense beauty, and environment from deep within the canyon just can't compare when you are looking down from the rims. And don't be deceived: Whether it's the north or south rim, be prepared for lots of people and crowded parking, especially during the tourist season. The ambience at the north rim was spoiled by a bar/nightclub in the village that played loud music. Very disappointing. Ironically, the drive to the north rim, I thought, was the best part. Running the Colorado river remains my most memorable trip and Grand Canyon experience.
Pros: Geology, the Ecosystems, River history
Cons: Crowded at rims