We read about this before we went to Brussels. We thought, "That seems pretty cheesy!" We found ourselves curious enough to go check it out, though!
The Atomium isn't in the centre of town, you have to take the Metro outside of the main part of the city to see this amazing giant Iron molecule--it is on the northern outskirts of the city. It was built for the International Exhibition of Brussels in 1958 by André Waterkeyn. It has nine large spheres with tubes linking the nine spheres and stands 102 meters (about 334 feet) in height. There are escalators inside the tubes that take you to the different exhibits housed inside the spheres.
There's tons of information about the Atomium and its history with pictures, information about EXPO 58 (a.k.a. The Brussels World Fair) at its official website. Find this at Atomium's Website.
We arrived by Metro and once we were out in the open, we found it to be much larger that we had imagined it would be. We purchased our tickets and waited our turn to go up into the exhibition halls. (There was some cheesy man dressed up as a cheetah or something -- we really couldn't figure out what it was -- that you could have your picture taken with while you were waiting to go up. NO THANKS, WE'LL PASS!) You could purchase tickets for other local features such as the IMAX theatre next door or Mini-Europe (I'll explain this in a minute). It was 6 euros each to get into only the Atomium.
You will find a permanent display of items found back in the ‘50s--during the time the Atomium was built. The other displays are rotating exhibitions. It was like stepping back in time visiting the ‘50s exhibition. I was born in 1974 so I can't say it was first-hand knowledge of the way things used to be. You could find fine examples of furniture and games, toys, and articles of history from that time.
We didn't go into Mini-Europe, but we had a great view from inside the Atomium. It is essentially a park where all of the famous sites of Europe are scaled down so you can walk among them. You will find such things as the Eiffel Tower, Big Ben (St Stephens's Tower), the Berlin Wall, among many, many more. It has quickly become a very popular tourist attraction in Brussels.
If you have children or just some extra time to kill, I would recommend you venture out here (Take Metro 1A, the yellow line, in the direction of Heyzel). Otherwise, I would recommend just seeing some of the other major sights within Brussels as we didn't find it to be all that exciting for the time it took to go out to see it.