I have been to the Observatory a few times, sometime for brunch, and sometimes just for a casual drink.
Brunch on the 52nd floor is an experience. The restaurant covers the entire floor, so you can look out all across the marina, and Palm Jumeirah. The food is well spaced out, and all tastes good. I didn’t find there was as much selection here as other brunches for a similar price. Last time I went it was around the AED 500 mark. They did have a great drink selection, and I really enjoyed their champagne cocktails.
The service is very good, and waiter come round and make drinks for you in front of you.