I traveled to Belgium in 1999 on a whim, for a long weekend in Antwerp, with about three days' notice. It was the best trip to Europe I've ever made. First, let me give you five (or six, or seven) really good reasons to go to Belgium and Luxembourg.
1) Since they are somewhat off the beaten path (with the possible exception of Brugge (Bruges), in Flanders), they're cheap. You can get top quality accommodations, food, and beer for about half what you would expect to pay for similar quality in Paris, Cologne or Amsterdam.
2) If well-preserved, but still functioning, medieval towns are your thing, consider this for a moment: Antwerp, Brugge, Ghent, Namur, Mechelen, Leuven, Tournai. Any questions?
3) The best beer on earth, period.
4) Small enough to get anywhere within two hours, and entirely realistic to tour on a bicycle.
5) More linguistic, cultural, artistic and geographic diversity crammed into a small space than any other country in the world. You get Dutch, French, German, and maybe even a little Spanish.
6) A courteous and helpful service ethic. Belgians love tourists! And I love Belgium!
7) Jacques Brel. If you don't know, make it a point to discover him while you're there.
And, of course, there are also five good reasons to buy this guide rather than other guides.
1) Essential historical, cultural and literary contexts that explain these small countries' somewhat confusing, multi-ethnic histories.
2) Encyclopedic coverage of all the cities, towns and important natural sites, including insightful historic and practical information.
3) Immediately up-to-date listings. Be sure to buy the most recent edition.
4) More substance than style. Rough Guides are intentionally that way.
5) A "not to miss" section that is well researched and looks beyond the obvious.
With mighty Brugge right next door, I still think incredible Ghent gets the short end of the stick. Some of the writing is a bit stale in this regard. For me, the hoards of tourists in Brugge make the experience in Ghent that much more authentic. There's nothing like falling into a Ghent pub at about 4:30PM for a Duvel, and staying on until you forget where you are. It's awesome. The same goes for "so-beautiful-it-hurts" Tournai, in Hainaut.
If you don't buy the book, go anyway. And if you go to Belgium and spend all your time in Brussels, shame on you:)