Eat here if you're not in a rush. Otherwise, I found the burgers to be delicious. Meat was of quality and wasn't oozing with oil, which makes me not feel so guilty. They also have a variety of healthy options. Whatever you do, get any of their fries and try them with any of their aioli sauces. The garlic aioli was so good with my fries and went well with my burger as I am not a ketchup gal.
Service - it was nook on a Sunday and it was barely half filled up. Service was way slow and it took even longer to get our food. Waiters were nice though.
In all, wouldn't go out of my way for Burger Bar but if I were staying at the Mandalay Bay or any of the conjoining hotels, I'd probably stop by again.