The museum was built from 1917-1920. It holds over 5000 objects including Angkorian statues, lingas and other artifacts. The bulk of these items were protected from being destructed during the war. The building was dedicated by King Sisowath in 1920 and inhabitated of thousands bats.
It is fantastic from the outside, the roofwork is very intricate. If you've got the time in the city it is worth a visit, but not a top attraction in my opinion either. Consider visiting this after Wat Phnom, Royal Palace and other attractions.
It looks really nice from the outside. We were really disappointed from the inside though. First you can't take pictures and then there is not really much to see. Definitly not worth the money and time.