I have been here a couple of times, and the bar seems to be quite empty on working days till 19:00,…read morebut pretty much overfilled on Fri and Sat evenings. So, you might want to make a reservation on those 2 nights.
The view is indeed fantastic, so if you are looking for that - it is a great place to be.
However, do note that the drinks are somewhat overpriced, e.g. a coffee costs 4.5€ which is clearly above Dusseldorf average in terms of price, but is nothing special in terms of quality.
Overall: awesome design and lights, neat waiters, wonderful view, but drinks do not offer a good price-quality ratio. I have never eaten here.