The WM is MIGHTY
The WM is quiote mighty at high levels, although it is not necessarily that overpowered if you compare it to spellcasters. Really, the PRCs for fighters that are called overpowered are the only way for them to be competitive with spellcasters at high levels.
The problem with the human fighter as written is that they wind up with 20 feats by level 20, and there is no one style (except OA martial arts, where a Monk3/Fighter4/Shintao Monk10/ 3 levels of your choice can dominate, but not much else, esp a single calssed MA fighter) that can really utilize all of those feats. A fighter peaks early, and gains little benefit from mastering more than one style (besides, it doesnt make good RP sense).
That is why the Weapon Master, the Order of the Bow Initiatate, the Duelist, the Cavalier, etc exist. They allow a fighter to continue in their initial path when they run out of appropriate feats. While such PRCs can be altered, they are vital to fighters' viability at high levels.
That being said, in my 16-17th level campaign, we have a Weapon Master who does indeed dominate physical combat, but not necessarily more than a fighter of that level should.
That being said, the class takes advantage of some the most munchy points in the 3e system.
It directly improves upon the Whirlwind/Great Cleave combo, allowing it to be used as a standard action, and thus more often. By this level, though, everyone will be hasted, so they will get that movement, anyway. What it does do, however, is to allow the haste action to be used for an extra attack on one of your targets. This, along with the max damage and ki critical a few times a day, lets the WWA kill-all work for creatures that the WM can kill in two hits instead of just one, becuase that attack will set in motion a Great Cleave chain that lets him finish off those around him.
It also provides a rare opportunity to get an improved critical that stacks with both the feat and keen. Be aware that WMs in many common weapons, particularly swords, which tend to have 19-20 (short sword, longsword greatsword) or even 18-20 (scimitar, falchion, rapier) crit ranges, will threaten on a 9 or a 13 respectively. the impact of this depends on your agical item system. If PCs can buy or comission items easily at normal prices, then Keen is easy to get (altohugh the spell keen edge, along with greater mag weapon, lasts pretty much all day at high levels), and Vorpal (the real problem) and burst can be added, too.
In practice, this makes lackies bait for this PC. IMC, I generally make sure to use alckies occasionally just so this PC gets to do his thing and look cool. Like any fighter, he is no match for an intellegent spellcaster, but he also shines, but by no means unreasonably, although that might be because he wont ever get a vorpal sword, against large monsters or other fighters. The latter, of course, is something of a good thing. the party SHOULD have a fighter that can counter baddie fighters, and you can always appropriately manipulate the baddie fighter's power.
So while there are a few things to watch out for (Dont allow him to eventually dual-weild vorpal weapons with 18-20 initial crit ranges), I am very happy with the P class IMC as-is.
