I'm currently playing one, and enjoying the heck out of it. I'm going to do a rundown of their abilities and why I like them.
Pain Points- This is a nifty ability, but not spectacular. The bonus on critical confirmation is handy. The boost stunning fist DCs is certainly a good thing. The boost to quivering palm DCs is downright amazing when combined with later abilities.
Martial Arts Master- This is a big deal. This lets the monk take fighter only feats. Greater weapon focus and both weapon specialization feats do a lot to help a monk's accuracy amd damage output.
Exploit Weakness- Oh boy, this one here is a hot button. Some people think its terrible, because it requires a roll and swift action to use. Others think its too good because its an at will ability.
Personally I fall somewhere in the middle, but that opinion may change after more playtesting.
What it does: It allows you to do one of two things as a swift action.
1) Make a wisdom + monk level check against a creature or object to get a bonus to hit and ignore DR or hardness of the target.
2) Get a bonus equal to half your monk level as bonus on sense motive and a dodge bonus to armor.
It only works on a single target at a time.
Note: Technically this second ability does not ask for a wisdom check. For those who think the check makes the first ability useless, this could be a decent fix.
Extreme Endurance- Thia eventually makes the Martial Artist immune to: fatigue, exhaustion, stunning, energy drain, and death affects.
This ability is very cool, and for those who've wanted to use monk and barbarian together its a godsend. Immunity to fatigue means you can rage-cycle at will with no ill effects.
Physical Resistance- This gives a scaling reduction to all ability drain and damage. Very cool for avoiding debuffs.
Improved Pain Points- This is not in the book, but the writer said that abundant step should be replaced with an ability that doubles the affect of pain points. Spiffy.
Defensive Roll- As the rogue talent of the same name. Gives you a chance to not die from physical attacks when you otherwise would. Very useful for a front liner.
Quivering Palm- A martial artist gets additional uses of this ability starting at 15th level. Remember how pain points and improved the DCs on quivering palm? Yeah, now you get to do it more than a standard monk. Good times.
Greater Defensive Roll- As the rogue ability. Better odds at not dieing in melee. Its a good thing.
All in all these abilities add up to make martial artists hit harder, more accurately, and take hits better.
Does this make then equal to an optimized fighter or brbarian with a greatsword? Probably not. But does it make them a viable front liner? If they can get a guided weapon, I think so.
Wait, whats a guided weapon you ask?
Its a +1 weapon property from a paizo splat book that allows you to use wisdom instead of strength for hitting and damage. And I think its absolutely necessary for monks to compete in a front-line role.
Its up on the PFSRD. Ask your dm if you can use it.
This concludes why I think the martil artist archtype is amazing.
Thank You
-Cooperton