If you cant use it until level 5, dont give it out until level 5. Problem solved, and the mechanic (which I happen to like) still works.
I also like Rage Strike, but not giving it until level 5 would be worse. It's a class feature. This isn't the era anymore where you need to level up in order to get your class features; you have them all at level 1. If it's something you get later it's a power, feat, or PP/ED feature.
Now that said, I think Rage Strike is absolutely fine on every account. It's simple, effective, an innovative way of breaking out of the default striker build, and it doesn't discourage people from going nova anymore (which every other class can do).
And the mighty barbarian is: overcomplicated and fiddly?
There is rage(strike), there is needing to remember when to get your temp hp, there is needing to remember your extra crit attack....
...by themselves, ok, but put them together, and Conan's head starts to hurt.
I think the Rage-At-Will is ment to spent the rage (so you are no longer in that "stance") you are in, instead of expending another Daily.
So if you are raging you gain your typical bonuses from that daily until you decide to put your whole (leftover) rage into one strike but are exhausted afterwards (rage ended).
This would make more sense IMHO since you could use it from level 1 onward and would give you a meaningfull tactical choice (much damage now or rage bonuses for a few rounds more...).
Since my post got ignored (besides Jack99 ... thx) I wanted to repeat my suggestion since I think that this would clear all the problems with the Rage At-Will:
Since my post got ignored (besides Jack99 ... thx) I wanted to repeat my suggestion since I think that this would clear all the problems with the Rage At-Will: