If you are a true car/motorcycle lover this is a place you must visit!
Worth visiting for sure but would not put on the top of the list if you are visiting Brussels for only a couple of days. There're other interesting sight-seen to do in this fantastic city but if you've done all the 'classic' visits this museum should be the next on your list!
Just in front is the Army museum, full of aircrafts that is also worth visiting. I advise you to consider visiting both in one afternoon. Open on Sundays also, which is an absolute plus.