Heard about Mastro's for a long time and finally had a chance to try it in Vegas.
This location is insides the Crystals @ Citycenter. If you go to this location make sure to request to sit in the tree house (it's the giant wooden structure you see in the pictures), it's much more interesting then just sitting in the regular dining room.
The service was impeccable; water was always full, the plates and crumbs were constantly being cleared and the waiter was friendly.
Me and my wife shared the double cut porter house. This thing was REALLY good, and was probably the best steak i've ever had at the time. We were not even hungry and we ate half of it. Took the rest to go and the next day i just straight up ate the leftovers straight out of the box (no fridge or microwave in the hotel room).