Generally speaking it is not OP.
But from BAB 1 to BAB 3, as long as you were already getting a +5 to Strenght (something I usually do) and if you already had a combat feat waiting for its purpose (like playing a fighter), then it is definitely worth it.
If you are into giant weapons it is the best, one step ahead from the next best options (medium sized Bastard sword, 2d8) and two steps ahead from most (greatsword, 2d6, a medium sized dagger, 2d6, etc.).
If we are talking about weapons in general then I would put it at the same place as the best of them, not above (it´s crit is second level and you need a feat).
Tax-wise however, and if you are using it for damage, it is better than power attack until BAB 12. I usually consider Power attack as the basic standard of a good combat feat.
+1 to hit is more or less equal than +2 to damage. This thinking that you are hitting half of the time (rolling an average 10.5 in your d20). If you are hitting nothing then the +1 to hit is stronger, if you are hitting with a 2 in your d20, the damage is more important.
That means power attack two-handling a weapon gives you a net profit of +1 to damage +1 per 4 BAB until you are BAB 20 and get +6 to damage,but with all seriousness, the further you advance the easier it gets to hit with your first attack so it would be a bit more in late game.
In conparison, the one feat invested in getting your scary axe nets you an astounding +3.5 of average damage at level 1 with no to hit penalty, making it absolutely better than power attack from BAB 1 to 3, better from levels 4 to 7 and probably still more worth it but it depends of your campaign and character from 8 to 11. Anyway is better than greater weapon specialization.
Pumping your size to destroy everything:
- Titan fighter (horrible archetype)
- Bigger race allowed (probably a medium one)
- Enlarge person permanent (yay, now you are Large)
- Buy a huge axe
- Get impact on your axe
- Your axe deals 8d6, an average of 28. Probably not worth the s@$@ty archetype.
- Get the full line of vital strike. Now it is 32d6 or 112 of average damage at level 16.
Does it fare well?
Full attacking is still better as long as you get 3 hits in. Let´s assume you are hasted thanks to your boots of speed at every turn by level 16. So hitting thrice should not be difficult.
Obviously that move action that vital strike saves you is a great asset against squishy enemies or enemies that move around or if you got the ability to drink potions as a move action as well, etc.
Still, if you are not taking that s+*!ty archetype you can still get to 6d6 which is pretty nice and that one feat would have net you 7 points of average damage in conparison to the same build with a gratsword.