My wife and I stopped by The Viking Planet when we had some time to kill after we went on a sightseeing cruise and we're so glad we did. We started by doing the little virtual reality game which was excellent, and the entire downstairs museum was awesome.
It's a little different than your normal museum as it's more of an 'experience' than a place you just go and look at stuff. The interactive elements were really fun, and overall it was just a really cool experience. We weren't able to visit the viking museum while we were in Oslo, but Viking Planet actually had a 3D tour of the exhibit which we were delighted to discover. Also, the staff (Markus and Christina) were awesome and so kind.