It was a little bit hard to find the entrance, so you may want to take a good look at a map, and keep an eye on the signs. The building is facinating.
There is a small gift shop which sells items which were stocked in the hosptial/bunker years ago. These are very reasonably priced and somewhat unusual (eg bags, gas masks, canvas stretcher).
The tour guides have a lively spirit about them, and spoke very enthusiastically.
The guided tours are hourly, so if you find you have just missed a tour just walk up to the castle area and wonder around the streets or pop into one of the lovely coffee shops.
It is a bit chilly in the building, so if you think you will be cold they will loan you a fleece. These all looked smart and clean - but I did not need one.
I think some people might worry that it would be claustrophobic, but I did not feel this way at all. The rooms are well lit, and well ventilated, and of reasonable size. I guess you could look at some photos to guage your own personal view.