I've used the Eyewitness Top 10 guides to a number of cities, this one most recently. The series is well designed; it can genuinely be fitted into a pocket, has good maps and is not heavy.
Being Top 10 Guides, they are not intended to be the most detailed guides on the market. If you are travelling only for a long weekend they will provide all the practical information you are likely to need and more in the way of activities than you could possibly do at about half the price of the bulkier comprehensive guides. If you are staying longer, then a comprehensive guide may be what you want.
With regard to this volume specifically, I had two days of playing tourist in Brussels and it served me well - I still got lost three times in two days in the maze that is Brussels old town, but not so severely as to feel stressed...whether another book has more detailed maps of the very centre of Brussels I can't say. A seperate Metro/Premetro map would have been useful, too.
I was able to plan my "hit list" of activities easily and even found a much needed English language bookstore...it's a branch of Waterstone's.