Giulio

The website with information on the tours is not available.
You can just go there and they have a tour every hour from 10 a.m. (at least on weekdays). The tour costs 6€ and lasts around 45 mins.
Unfortunately when we were there they were testing the lights so we could see the main stage properly.
However the rest of the tour is really nice and the guide gave some interesting information. Especially the upper floors are beautiful.
Please note that the tour is available only in Italian (at least when we went) - the guide allowed me to translate for my wife who doesn't speak Italian

Absolutely good experience.
I have been here maybe 15 years before but I was well impressed.
I admit that we were lucky and got one of the best tables at the terrace. I recommend booking in advance because the terrace was fully booked on Saturday already a day before.
The place is well kept but not too formal, the obvious highlight is the terrace with the amazing view on the city.
The service was really good, fast and attentive without being intrusive, and at least some waiters speak English.
The food is also quite good, quite a twist on typical German foods. Also a wide range if vegetarian and vegan options is available.
Prices are in line with the quality, medium-upper range.
Will be happy to come back in the future.