There are 3 locations of Starbucks in the Luxor, and I've been to two of them. One of them is near the lobby. That location is absolutely horrible, since the lines were long and the baristas there were extremely slow. There wasn't even a place for you to sit at all since that small location was completely filled up.
I also went to the food court location, and even though some long lines existed, the baristas weren't as slow as the other location. And since there's plenty of seating in the food court, you don't have to worry about getting a seat to enjoy your coffee and pastry.
The prices were high, but not as high as in Mandalay Bay. And they do accept Starbucks Cards for purchases at both locations.
Do what you have to do.