We visited Peace Osaka after going to Osaka Castle (about a 10 minute walk through the park). We used our Osaka Unlimited 2 Day Pass , which is 2700 yen and includes attractions, subways and buses. This museum was the first one I've been to in Japan where Japan openly places the blame for it's own suffering and the suffering of other nations during the period from the Manchurian Incident through to the end of the Second World War squarely on itself, (as a result of it's own aggressions). It also acknowledges war atrocities and other crimes against humanity. The theme of the museum is that war is terrible and brings suffering.
The first very effective exhibition is about the incendiary bombing of Osaka during WW2. It tells the story, in Japanese and English, effectively through war footage and personal accounts. The second exhibition is about Japanese aggressions in other Asian countries. The third exhibit was a temporary installation with no English, but it seemed to be related to WW2.
The Peace Osaka building is architecturally interesting, but the building and the exhibits are looking a bit tired. I would recommend the museum to history buffs, but not to families - it's too confronting for kids.