Fatburger and In-N-Out are my favorite burger joints in Vegas. Fatburger is more expensive but they have more variety in their menu (yes, I know about animal style and protein style).
The key trick for Fatburger is to get a discount card. They punch it for every burger you get and when it is full, you get a freebie. However, you have to ask for their card because they don't volunteer it and they don't ask if you have one.
Skinny fries are better than their steak fries. Chili is pretty good with chili cheese fries a favorite side. Onion rings are very good too. The milkshakes are real old-fashioned shakes (similar but larger serving size to In-N-Out.)
If you eat very late, they are your best choice since they are 24 hours. Sometimes you have to make some noise if they are in the back instead of manning the register, however.
They are made to order so you can get extra whatever. The one thing I don't like versus In-N-Out in addition to being more expensive is that they use chopped onions. You can get a fresh slice at In-N-Out and slices are better than chopped onions because there is less cell wall damage and water leakage.
This location is not as good as some of the other Fatburgers but a mediocre Fatburger is still better than the best McDonald's.