I recommend visiting off-season because during school holidays there I can be a long queue.
I would allow for three and a half hours for a visit, so you can visit the cafe, and take time to read the large amounts of information in the Churchill Museum.
The attraction is the former command basement that Churchill and the Chiefs of Staff used in the run up to and during World War Two
You walk along a clearly defined route looking at the map rooms, communications rooms, private bedrooms, conference rooms etc.
They are exactly as they were during the war, with a few wax models of soldiers added in.
The highlights are Churchill's bedroom and the Map Room.
In the middle of the site is a large detailed room called "The Churchill Museum".
About his life, politics, personal life and humour.
There is so much information and so many artefacts in there that it needs an hour and a half on it's own to do it justice!
You will want to take many photos, though I recommend the compact "Secrets of Churchills War Rooms" book which is an expanded guide book at an affordable price, stocked with photos and relevant information.
There is a very well stocked gift shop.
Visitors get given an audio guide which is essential, as the site is large with so much to see and learn.