Firstly a confession; I have lived in Brussels 7 years and am very ashamed to say that despite this museum being in comfortable walking distance this was my first visit - however I will certainly be back!
Entrance fee for permanent collections is, at time of writing (Jan 2012), €5 for adults but I paid €8 in total to see also the Sabena exhibit which runs until the end Feb 2012 - which was excellent value.
Be prepared to be overwhelmed as in summary there is a vast array of collections to see ranging from pre-historic to modern eras (Roman, Ancient Egypt, Iranian, S.E Asia, Islamic Art, Middle Ages, Native Indian, African etc etc) and exhibits cover a huge sphere of artifacts (paintings, tapestries, metal wares, glass, scientific instruments, furniture, woodwork etc etc).
The museum has a faded charm which some may dislike but I felt it added to my experience.
Also be aware that the museum is very large and split across many halls, floors and levels - I almost missed the majority of the museum exhibits not realising there were many more halls, floors and levels after the museum shop!
Would strongly recommend the Cloister & Roman areas as both were visually impressive.
Missed out on excellent rating as the description of artifacts was in many cases brief, and some areas are in need of tender loving care beyond the faded charm I mentioned previously.
Overall to do the museum justice you need to allow lots of time but there are plenty of toilets, a cafe (which I did not try) and a large, slightly disorganised museum shop.
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