I am currently playing a Warpriest at 8th level (started at 5).
I chose to go with the Champion of Faith Archetype since I wanted smite and smite is awesome.
Since it came out I have found both offense and resources wise, the inquisitor is a better class.
A better WAR priest.
The Inquisitor spellist has early access to some awesome spells well before the warpriest.
Deadly Juggernaut for instance and gets some brilliant spells he can't cast at all like Righteous Vigor and Litany of Righteousness (which an inquisitor can't use anyway since he doesn't get an Aura)
"But he gets full cleric spells", you cry.
I know. And because of the 6th level casting gets it way later than the cleric.
This is HUGE. The cleric list is ok but getting stuff 2 levels late makes it suck.
I am not one of those who scream that they should have kept full BAB, but considering the closest Parallel classes (Magus and Inquisitor) are less intensive for resources the class lags a bit.
A warpriest has to blow a fervor AND a spell to sswift buff+Attack.
A magus can cast+attack all day long via spell strike. Spellstrike doesn't cost anything. Spell combat does a little (-2 to attack) but the cost is negligible considering and Attack+Haste is far more game breaking than Attack+Divine favor.
The right Battle control can trounce an encounter.
Vs a Inquisitor well Judgements as a swift doesn't even cost a spell at all. At low levels a spell is better (point for warpriest) but as soon as second judgement comes online, freely picking +'s to attack, damage or saves FOR ENTIRE FIGHT can tip the balance the other way.
I don't necessarily think there is anything wrong with the warpriest mechanism but are strongly feeling Paizo in their efforts to not create any more Unique spell lists could have said
Spells:The warpriest uses the Inquisitor and Paladin lists for spellcasting.
That would grant a Much more offense focused list (remember WARpriest) and a better healer as well. with adding stuff like Righteous Vigor, early Deadly Juggernaut, early lessor restoration, Litany of Righteousness etc. Freaking Holy Sword.
It's game changing for the class and better fits the theme.
As for what it's good at I have read Undone's guide and agree with Most of it but have had lengthy back and forth that Warpriest's make pretty good Intimidators and with their bonus feats the combo of
Cornugon Smash, Shatter Defenses
and a Sickening Weapon
To drop Shaken, Flatfooted and Sickened is a pretty potent trick to debuff enemies.
They also make Tank builds (Fullplate, Shield, Ioun Stone, Sacred Armor) whhile still doing great damage (Smite or Destruction Blessing)
Still that damn cleric list bothers me.