I would not go out of my way for this. Turns out Ethyl M and M&M Mars are related. The tour agency we went to Hoover Dam with made this a stop on the return to the casino.
You are taken into the factory, or what appears to be a factory, and walk on a catwalk above the floor. The catwalk is glassed in so you can look down on the workers and machinery.
We saw lots of stainless steel ovens, etc. and a few workers. If the factory was real, it wasn't in full production. After you walk through (about 150 feet), you wind up in their store where you can purchase Ethyl M chocolates and souvenirs.
The best part of the visit was outdoor gardens. They have a great desert garden showing cacti and succulents from around the southwest. While it was beautiful, the cacti were all filled with tacky Christmas decorations that didn't really quite fit with the plants.
However, seeing the plants in their natural habitat made me overlook the rest. That made the stop worth it.