I visited this site in August 2015, together with a friend who lived in Norway then, and I haven't much to add to Els's review about the WHS itself.

But if you haven't seen it, watch the tv-series The Heavy Water War (Kampen om Tungtvannet) (shortly) before going there. Of course, it's dramatised and not 100% accurate, but it's a nicer way to understand what happened here in WW2 than the museum can show you. It'll help you to breathe in the region's history when you're walking in the valley or across the hills. Perhaps that's what made me give it the extra half star.

(The picture is showing the Sun Mirrors that Els already mentioned; funny enough of it's own to watch from a nice terrace below.)