With the current wording of Devastating it allows a warrior with a weapon with a x3 or x4 critical hit multiplier to do more damage with a single standard action than they could with a full round action (Backswing). The warrior can use this feat every round.
It's an expensive, high-level feat, and it should be awesome. But not quite this awesome.
The feat needs some type of limiting factor. So here are three thoughts on going about it:
1.) Instead of a critical the feat just does double damage (extra damage dice not multiplied, per usual). In case of a critical add multipiers normally.
This brings the damage under the average damage for a Backswing. It also opens the door for powers activated on a critical.
This is my favorite option.
2.) A combat feat cannot be used in the round proceeding a Devastating Blow.
This adds a small risk/reward to using the feat, since it hampers your next turn. That scythe is still awfully impressive.
3.) Devasting Blow switched to a full round action.
It would still a worthwhile feat, because you'll do better damagge with a greataxe or way better with a scythe. Greatsword wielders may waver, because the damage is about the same or less than Backswing, and it really only gives an advantage when facing DR that you can't bypass, or if you have an advantage on your next attack roll, but not the ones after it (invisibility). On the downside, one of the aspects I liked best about this feat was that it was a standard action.
And before anyone posts that Devasting Blow can't possibly be as potent as I'm suggesting, here's an example using an 11th level barbarian (base strength 22, greater rage bringing it to 28) and the average damage he deals each round against a target with the given AC.
Scythe regular full attack
AC20 47.61
AC25 34.155
AC30 18.63
Scythe with Devastating Blow
AC20 68.4
AC25 57.6
AC30 39.6
Scythe with Backswing
AC20 56.951
AC25 40.779
AC30 21.761
Greataxe full attack
AC20 49.335
AC25 35.393
AC30 19.305
Greataxe with Devastating Blow
AC20 55.575
AC25 46.8
AC30 32.175
Greataxe with Backswing
AC20 59.759
AC25 42.785
AC30 22.799
Greatsword full attack
AC20 50.6
AC25 36.3
AC30 19.8
Greatsword with Devastating Blow
AC20 38
AC25 32
AC30 22
Greatsword with Backswing
AC20 61.52
AC25 44.044
AC30 23.46