We went years ago to the one on the first level. Not bad nosh tbh!
I have to disagree with downton about the levels though. You have to go to the top, especially at night when it's dark, you can barely see and you're petrified of falling. A friend and I often spontaneously burst out laughing at the memory of me up there. Sobbing because I slipped (and didn't even fall over) I thought I had died.
The first level was quite high enough in my humble opinion!!!! We went up in the evening and it is gorgeous all lit up with twinkly lights. Link for online booking. Jules Verne prices are a bit eye watering.
Yes you need to book. There is a separate entrance for those with dinner bookings but once you are up you are free to do as you please and wander round. It doesn't include going up to the very top level though as the restaurants are at the 1st 2nd stage.
Not worth going to the top IME as the views are the same from the lower level, just not as high.
I booked for dinner in the restaurant on the first level when dh was 50. It means you don't have to queue up. It was a very nice experience but not so posh really. If you want to go to the Jules Vernes place on the 2nd level, I think you need to book at least 3 months in advance.