We were very impressed with how the bus service was set up.; and it's FREE. Most of the bus drivers are quite friendly and personable, offering a little park info if needed, telling you if you're on the wrong bus, or giving a little friendly patter over the bus PA system. The bus service is essential as part of the park road system is closed to traffic a majority of the time.
There were a fair amount of people at the Grand Canyon when we were there in April, and I could see what a madhouse it could be in summer. We literally left the car in the Yavapai Lodge parking lot for 3 days where we stayed and took the bus everywhere. There are also bus stops at parking lots as well, if you aren't staying in the canyon.
The bus service is divided into three different lines: The Red Line, for Hermit Road; the Blue Line for Grand Canyon Village and area, and the Green Line for a few points on Desert View Drive. For hikers, there are bus stops near the popular trails, such as Hermit's Rest, Bright Angel, and the South Kaibab. It stops at the viewpoints so you can get on and off at your convenience, or if you want to stretch your legs, walk a km or so to the next stop.
It runs a majority of the time every 10-15 minutes (for complete info, read the little park newspaper you get when you go through the gates) and I found it very efficient.
If you are interested in hiking Rim to Rim and need a way to get back to your original starting point, then there is a company that runs a bus between the two. The trip is about 4 hours and the schedule when I went in October, 2003 was that it left once per day. Also, the cost when I went was $65 oneway.
Grand Canyon National Park provides a FREE shuttle bus system on three routes on the South Rim: the Village Route, the Hermit Rest Route, and the Kaibab Trail Route. No tickets are required. Simply board the brown and tan or white buses at any bus stop marked with a bus symbol on the map. The shuttles provide transportation; they are not tour buses. You must get off the bus to see the canyon. Shuttle service is not available on the North Rim. The Hermits Rest Route is NOT in service between December 1 and February 28. During those three months ONLY, Hermit Road and the road out to Yaki Point/South Kaibab are open to private vehicles.
Various touringcarorganisations offer tours along both (West and East) Southrim-sides. They connect the several outlookpoints and along the Westrim (in highseason) only these busses are a way to see them all (unless you make the walk).