I visited Europe on business for about 10 years and went to many major European cities (Brussels, Amsterdam, Paris, Barcelona, Madrid, Copenhagen, Stockholm, Lisbon, Geneva, Vienna, Hamburg etc etc etc)
Now I dont wish to upset SteveD who has already replied to you, but I found Brussels rather disappointing and dull.
Because I was on my own on these visits, and often had free days at the weekend, I tended to walk round the cities a lot.
But I never found many areas or buildings in Brussels that really inspired or interested me, and I found many of the older buildings rather dirty, as though they had not cleaned the stones for years (they may have cleaned them since).
Given the choice I would far rather go to Paris, Barcelona, Madrid, Amsterdam (or most of the other cities I visited) than go back to Brussels.
Even if you have been to Paris before, go there again as there is more to see in Paris on a second visit than Brussels on a first visit.
Sorry SteveD, I realise I may have annoyed you, and feel free to defend Brussels if you wish. I have no axe to grind, just saying it as I found it.