I had, and still have, mixed feelings about the Stronghold Builder's Guidebook. How would a book that is so integral to any long-term campaign shape up, especially in the d20 system? Will it be full of details, or will it be a lazy attempt that will leave DMs wishing for more?
It turns out that it's a mixed bag, but with more good than bad points. (As a quick note, I am going to, at times, briefly compare this book with it's 2e incarnation, the "Castle Guide" - the only other book in D&D to detail how a stronghold is to be built.)
The Stronghold Builder's Guidebook is 128 pages, looking much like the class books put out by WotC. The interior art is sparse, which at first seems good (more space for details and maps), but could also to be constued as being very dull, sparking nothing for the imagination. The 2e Castle Guide has a couple of very nice full-colour pictures of different strongholds. This is purely a matter of taste.
There are 4 chapters, with Chapter 2 being the longest.
Chapter 1: A brief step-by-step guide in building a stronghold, to be used with Chapter 2 (the actual components). Quick, to the point, and easy to follow. Useful tables cover subjects such as terrain the stronghold is being built on/in, and the distance from a major population center.
Chapter 2: The meat of the book, and a massive chapter. All the components (well, most - I'll talk about that later) you need to build a stronghold is found here, including size, price, and description. Examples include Stables, Alchemical Laboratories, Magical Laboratories, Libraries (including a spiffy rule set on how libraries enhance Knowledge skills), Common rooms, Kitchens, etc. Some sample floorplans for some components are included.
Also included is a full list of NPCs along with monthly wages - and more detailed than the hireling section in the DMG.
Further in the chapter are full statistics for walls (hardness, hps, break DC, etc) along with costs for different types (masonry, iron, mithral, bone, etc).
Finally in this chapter is the section worth the price of the book on it's own: magical enhancements ("Wondrous Architecture"). This book (FINALLY!) gives full details how to add magical effects to the walls, rooms, etc of your stronghold (it's based on the Craft Wondrous Item feat). You can now mimic a tower that all those insane wizards insist on building - and have the full rules to do so! No more DM headaches.
Chapter 3: This chapter is the least useful in the book. It gives basic "ideas" about what to consider when building a stronghold, who to hire, and the like. It's designed much like portions of the class books - the audience seems to be those who wouldn't know an RPG from a hole in their head, and like to use the phrase "KEWL DOOD". There is a bit of hope in this chapter, though, with a brief section on the mechanics of attacking a fortress. Very useful, but woefully incomplete.
Chapter 4: The final chapter details and maps four different strongholds. They can easily be dropped in any campaign.
The good:
Anything that details a complicated subject like stronghold-building is appreciated. This book will save a lot of DMs from jumping out the nearest window when a PC declares he/she is going to build a stronghold. Further, the details on the wondrous architecture is incredibly useful. Not only can you build a castle, but full rules for building *any* sort of structure - including dungeons. Yes - with this book you can actually grab a dungeon you created and ran years ago and determine what it would have actually cost to build. That's slick.
The not-so-good:
Now the mixed feelings. I consider this book "a valiant yet flawed attempt". There are large portions that should be hacked right out, and replaced with more valuable material. The rules for attacking a fortress were decent, but lacked detail. The 2e Castle Guide had a great section on rules for mining/sapping (digging under the walls), building trenches, etc. This book? Pretty much nothing. The 2e Castle Guide also had a great little section (and table) on events that could occur when building a stronghold. Nothing here. (And on a related gripe: no sieging or mass-combat rules. *sigh* I guess it really is Chainmail for us.)
Another problem is that some of the wondrous architecture is (3e parrot term - get ready

"broken". Some items seem to be priced far too low. For example, to create a permanent magic-dead area (non-dispellable!) of 400 square feet (or, in actuality, 4000 cubic feet), costs only 66,000 gp. In stronghold terms - this ain't much - especially when some stronhold prices are going far in excess of 2.8 million gp. [DMs - get yer fixin' pencils ready!)
Finally, putting together the stronghold components isn't cut and dry. How they are put together, the costs for building up and down, and what's included in the price and not included in the price isn't always clear. Reading Chapter 1 and half of Chapter 2 a couple of times may be necessary. There's still a lot of work to be down when getting the final design - time will be involved, so expect a gaming session (4 hours or so) to cover all the details properly. DM SUPERVISION ADVISED!
The end result, though, is that it's better to own this book than to not own it. Even if a stronghold isn't part of the future plans of PCs, many components of this book can give DMs ideas on what to put into the villains' strongholds, and even much of the wondrous architecture can be slammed into any dungeon.
An average book if I ever saw one.