I didn't care for it at all and will be exchanging it for something else.
There were just too many little things that annoyed me. It eventually wore me down.
1) Fighting Styles. There's no real good reason that the fighting styles are keyed to your wisdom bonus. It is obviously included as a limiting mechanic, but the end result is that most fighters will achieve 1, maybe 2 levels in any fighting style. You would need a 20 Wisdom (+5 bonus) to master all 5 levels of a fighting style. Why should fighters have to dilute their physical attributes to excel at fighting? Why should a cleric have a better chance to master a fighting style than a fighter?
2) Each time I saw, "A penalty to all CHA based checks," I winced. Example, the Fighter Assassin: "The fighter assassin has great difficulty in relating well to any other person, for he always assumes they will betray him at the first opportunity. He receives a -2 penalty on all Charisma based checks." Oh really? Animal Empathy? That's a Cha based check. Will my horse betray me? (You're hard core, Trigger.) How about Use Magic Device? Geez, I can't trust my scrolls and wands, either? This is lazy game design. You can't possibly account for all the Cha-based checks that will ever exist-- but you could at least account for the ones that DO exist and take the time to list the skills by name that would logically be affected. We obviously know what the writer's intent behind the rule was, but the implementation was lazy.
3) Two-Handed Power Strike. "This feat may not be used in conjunction with Improved Initiative..." So how's this work then? In 3e, initiative is rolled once, at the start of combat. So how do I apply Improved Initiative vis-a-vis Power Strike? Do I decide at the start of combat if I will EVER use the power strike during that combat? "No, I won't use this feat at all this combat, I'll keep my initiative where it is," or "Well, I rolled a 1, so at best I'll have a 5... Might as well start power striking..."
4) Many of the illustrations are layed out on top of the text, but the background is not transparent and the runaround isn't set properly. As a result, you miss a word here or there. Sometimes those words are important, for example the base attack bonus prerequisite for an Acolyte of the Short Blade (p72). Oops. There are others.
5) Two-inch margins. An inch-and-a-half of art and another half-inch of white space. This is becoming a Mongoose "trademark."
6) The DM will need to do some work to get the OMCS to work correctly in conjunction with magic. For more on this problem, look for the Quintessential Fighter thread in the d20 Publisher forum.
In the end, the book looks lazy and rushed. I respect Mongoose's aggressive release schedule-- we are all hungry for d20 supplements-- but not at the expense of quality.
Wulf