I went today and I have to say that was worth it in my opinion. Though I was never alive when he was, I was lucky to have parents who are huge Beatles and John Lennon fans, so I grew up around his music and message of peace.
For those of you in or going to Japan, the Museum is in Saitama, the stop to get there to be exact is Saitama-Shintoshin. That takes you directly to the building it's in (which is outside the station gates). It's open from 11am-18pm (6pm) but it's closed on Tuesdays. It's about 1,300 yen to get in (1,000 if you are a High School or College student, your school ID is required to get the discount).
Sadly, there is no pictures allowed in the museum (otherwise I would post them) but if you are ever in the Tokyo area, hop a train and head to the Saitama-Shintoshin stop, I thought it was worth it and enjoyed every minute of it.