It is worth to buy AT THE TOP Burj Khalifa ticket. The view was really amazing. I suggest that the perfect timing to climb up is between 7pm to 9pm because you can catch the day and night time view. It will give you the different feeling. Some more, it is indoor and outdoor. It is do not have time limitations that you can keep take photo on the top until you feel tired. Else, there the souvenirs shop in there.