Per week on Paizo's forums there are at least two threads discussing the Fighter class and what may or may not be wrong with it. In these threads there are common themes to proposed fixes. The Combatant in this product feels like it reflects a number of those proposed fixes despite it's insistence that it is not attempting to replace the fighter.
Essentially the Combatant is the fighter at it's core but with the following differences.
Good Reflex save.
4+INT skill ranks per level.
Acrobatics is a class skill.
Does not need dex requirement for TWF.
The biggest changeArmor/Weapon training is absent, and in it's place are Martial Art Styles. Martial art styles are combat styles associated with a number of feats. If you possess feats within a martial art style you have then you get bonus abilities based on the number of feats you have of that style.
For example if you have 3-5 feats of the archery style, you may ignore up to half your level in circumstance penalties to ranged attacks and stealth becomes a class skill. As you gain more archery feats you get up to 5 more abilities of increasing strength. Some of the feats are on multiple style lists so since you gain 3 martial art styles throughout your levels, you can get a number of extra abilities if you synch them up. The abilities themselves are either obvious numerical bonuses or something amazingly complementary. (like pounce) Overall these are huge boosts of power and versatility making the Combatant a pretty competent beast in combat.
The product also comes with a number of new combat feats and honestly, if you don't use the Combatant class you'll want to use the feats. A lot of them are amazing. A few feel very at home with a monk and a number I would call Must Haves.
Overall for the price of one dollar you're getting a lot. A spectacular class AND useable feats. Very useable feats. The price takes this from being a nice product to one that is silly not to buy.