I go to the Starbucks in the District in MGM Grand every day. I love the people who work there, and I'm there all the time so they know my drink. Prices are a bit of a pain in the butt, however the staff is fantastic. I always give a tip because they are good.
There isn't much seating, but that's par for the course in a casino, they don't want you sitting, they want you wandering.
What makes the Starbucks is the staff, and they are very prompt, they line up the drinks with your milk when they are busy, so they pick it up and steam it instead of spending the time getting the milk.