If travellling with children, I would rate 5 stars. Fun experience for them. About 2 1/4 hours or 2.5 hours each way. The train station stops at El Tovar Hotel/Restaurant for about 3.5 hours. Plenty time to take in the sights, walk the rim, see the visitor centers, and have a lunch. Then back on the train.
If all adults,I would give three stars (drive yourself would be much faster.
I have done this twice and enjoyed both trips and scenery.
Note: bring snacks and drink on the train. You can buy it on the train if needed.