First of all, I should note that I am concerned with content, content, content. Others have commented on the artwork in the book - I tend to try avoiding comment because in many cases, artwork critique simply reflects personal taste. I find artwork useful in helping me envision a creature, but as long as it is technically adequate, it does not concern me. The artwork in this case is technically adequate, though I personally do not care for the style.
Having said that, let me cut to the chase. Simply put, this is a great book. Why?
Technical Reasons: Every creature is well-balanced and "rules-legal"; I haven't found any glaring mathematical errors in the creature statistics. At this point in the d20 game, publishers should have a firm grasp of the rules before going to publication, and it is surprising to see that many still do not. This book does not suffer from these problems, and as such, gets good marks from me on that count.
Portability: While every creature has an excellent physical/social description, including such should be par for the course when publishing a book of monsters. Bastion scores extra points for including a section on "how to integrate this creature into a campaign" with each creature. This is a fantastic addition that makes the book extraordinarily useful, as it helps DMs quickly integrate monsters into their campaigns and helps adventures write themselves.
Originality: There are a lot of original ideas for monsters in this book - and the best part is that they are spread out among all the monsters - every one has its own feel, and every one has unique abilities or combinations. Contrast this with a work where one or two monsters seem to have the market for originality cornered - all of the neat ideas/gadgets/abilities are stuffed into a small percentage of the monsters while the rest seem... well... blah.
Substance: I noted very little white space, and the text was in a reasonable font. Except for the Table of Contents, a short Introduction, and the obligatory License/Legal pages (the TOC and Legal stuff was necessary and the Introduction expected), the whole 96 pages is devoted to the Minions themselves, with a short section on using a few of the creatures as familiars (a nice touch, I thought).
Variety: This book contains monsters of all CRs, of all types (beast, aberration, undead, etc.) and of all elemental subtypes (air, electric, cold, etc.). This makes it useful for a wide variety of campaign settings. In my view, it is highly important for a monster sourcebook to deal with a wide variety of CRs so as to make it useful for many different campaigns (since character level is probably the biggest difference among all campaigns).
Comment - on filling a niche: I prefer to see a product take one of two routes: either give the reader a large variety of all types of creatures for "general use" in a campaign world or provide a large variety of creatures within a single narrowly-defined niche (e.g., Legions of Hell). This is because the niche book will be useful for those interested in the niche, and others will know that the niche book may not help them as much as a diverse book. Similarly, the variety book will be useful for most, but for someone wanting to run a campaign exclusively in the niche, it will be apparent that the niche book will be more useful. A book that can't decide whether it wants to be "general" or "narrow-niche" is problematic. As stated, Minions does an admirable job of being a "general" book.
Favorite Creatures: I will give only three, but some of my favorites include...
*Glacier Beast - While some may think of this as "cold-adapted owlbear on steroids," I liked the presentation, myself. Particularly nasty is the creature's ability to fling opponents through the air... especially nasty if there are sheer drops or (as is likely in an arctic environment) ice-cold water around. It is a simple and elegant way to make the creature more challenging and make it substantially different from the "hugging" owlbear.
*Bone Sovereign - Started out as a run-of-the-mill undead skeleton until (ostensibly through exposure to strange magical/evil energies) it achieved sentience. This baddie can control other undead and merge with skeletons (and can spew them out later, if desired). The idea of a huge skeleton spitting out "baby skeletons" rather appeals to me, and its "unhallow" aura makes it a tougher foe than just a big skeleton would be.
*Vapor Bore - Wins the contest for my "favorite extension of existing rules" by taking the concept of Charisma drains a step farther. We know Charisma is supposed to be representative of a character's ability to impose his will (as opposed to Wisdom, which represents his personal force of will as opposed to the force with which he affects others). What happens when it is drained/damaged to 0? Well, obviously, even the most willful character no longer has the ability impose his will on anything (the will itself and the ability to impose it are different things entirely). The Vapor Bore drains/damages Charisma down to 0, then "seats" its own ability to impose its will into the victim by "implanting" a point of its own Charisma into the drained victim, thus rendering the victim an extension of the vapor bore's personality. A beautiful example of using a rule in an original new way!
OGC/d20/Legal: My favorite part about the book, though, is that the text (though not the artwork) is 100% OGC. That is HUGE because it increases the book's re-use value immensely. Thus far, publishers have been loathe to use other publishers' material due to concerns about what is OGC and what isn't. This means that you may see many of these monsters crop up in other publishers' products. It also makes it an invaluable resource for other publishers themselves. This, to me, is important in showing Bastion's commitment to the Open Gaming movement as some publishers try to publish as LITTLE OGC as possible, trying to "protect their precious content" rather than "grow the pot." WotC has released a TON of material as OGC, I don't think it's too much to ask for other companies to follow suit. The more material that is put into the common pot, the greater the diversity of ideas available to all. This leads to better products all around. That's my own little rant, though.
Conclusion: Simply put, this is one of the best additions to my 3rd edition library - and the more I read it, the more it grows on me. That alone justifies the cover price (which some find too high), IMO. I would compare it favorably to any non-hardcover published for the d20 system (and even some of the hardcovers aren't as good).
I vote with my wallet, and Bastion Press, with its original, high-quality, 99% OGC sourcebooks, will continue to see my business. That's the best endorsement I can give it.
--The Sigil