I'm sure this will be a "must visit" for most gamers visiting Paris. I've just spent 3 hours there whilst the wife and kids visited the tourist hot-spots, and I could have stayed longer.
Apart from the obligatory visit to Napoleon's tomb there are several fascinating collections. The section on Medieval and Renaissance armour is excellent, the part on "Louis XIV-Napoleon" fascinating but not very chronological, and I need say no more about Napoleon. If you are into the Franco-Prussian war that is well covered, and the WW1 and WW2 galleries very modern and informative. One section containing dioramas of French town and ciry defences was truly excellent.
A few things to note;
If you use Google maps to get there it takes to the wrong place! The entrance is from Esplanade des Invalides.
Both the cafe and restaurant were shut, as was the bookshop.
Most signage and caption boards are in English but the signs can be misleading
Still, well worth a visit