Let me start off by saying, this place is awesome!!!!
I had the filet medium rare and it was one of the best filets I've had. The Lobster Mashed Potatoes are a must. It was my favorite part of the meal. Huge chunks of lobster and creamy mashed potatoes. The creme corn was good as well, but I would go there just for the lobster mashed potatoes. If you don't try the lobster mashed potatoes, you are missing out on the entire experience of Mastro's.
As for dessert, the warm butter cake is amazing. I would recommend this or the creme brulee.
Warning, you will most likely feel like your stomach is about to explode when leaving there.
On a last note, get the lobster mashed potatoes.