It's a huge landmark of Brussels attracting more and more people every year. About 7 years ago when I moved back here, I used to go there every weekend, and walk around a very peaceful market. Nowadays, if the weather is good, it is super packed, but still with a nice laid-back atmosphere.
On the market you can find clothes, furniture, jewelry, tons of cups and plates and cutlery, and everything under the sun, basically. The prices can be outrageous, so be sure to not get too enthusiastic!