I have taken the bus a few times but you wouldnt want to be in a hurry to get there. The train is a lot faster and gets into central station in half an hour. The train costs about 8 eur. If you are looking for a really central hotel in Brussels, basic, very clean, but not luxe, then I would recommend Hotel Barry at place Annessens. Near the bourse, near O Reillys pub and within walking distance of everything central. Last time I stayed there I paid 45 eur a night (Jan). Very pleasant service and breakfast included.
kaygo