While we didn't go to the Grand Canyon on a bus we did go to the Grand Canyon twice through Look Tours. One trip we flew into the West Rim of the Grand Canyon from Las Vegas and rode ATV's from a working ranch to the Canyon. The second trip was when we fly into the Grand Canyon again from Las Vegas and took a bus tour from the Grand Canyon Airport to the Canyon. Both trips we great. We occured no problems or issues and enjoyed both trips.
We also used Look Tours to take 5 helicopter rides over the strip at night. Again we had great trips.
While the last time we used them was in 2008, we use them to book a tour everytime we go to Las Vegas. Check out their website they have tons of different options to choose from.