I visited in early June on a cloudy day. There are two distinct parts, the Documentation Center/Bunkers and the "Eagles Nest" (a nickname given by the Americans at the end of the war) to what the Germans name Kehlsteinhaus.
Eagles Nest (house and road) was built by one of Hitler's inner circle as a present. What I was told (it was foggy so no point to taking the expensive ride up): The road up is said to be an engineering marvel. The view at the top is nice. The Allies destroyed nearly the whole thing; a restaurant was subsequently built as a tourist attraction. There is a bit of the original building remaining. In short, the reasons to go to the top are the view and the road itself.
The Documentation Center is well worth a two-hour visit. It's very well done, explains how the town of Obersalzberg went from a summer resort to Hitler's HQ, including a film with interviews of some of the town's then-residents who remember the Hitler years and describe the changes over the years. It also documents in detail the rise of nazism, the various methods used, the insidious propaganda. Then there are the bunkers - something that needs to be seen, not described. This is anything but a tourist trap. No gift shop, no souvenirs, though there are some books for sale.
Get to the Documentation Center when they open. Find out the time schedule for the ride up to Kehlsteinhaus and back down again. The Center is on one side of the parking lot, the ticket booth and departure for the ascent are on the other side. (There may be a gift shop there). With a car, you should be able to do that in the morning and still have enough time for the Königsee.
Another overlooked "gem" is the Salzburger Frielichtmuseum, about 25-30 km from Berchtesgaden.