TBH they should add some status bar so people know when/how-often they can use it, something like this
https://imgur.com/a/ujFeIHT
Aaand - that’s in case if it works 
BUT, if it doesn’t ? (or as others have mentioned Crit overshadows it too much), then it needs some more love late game, have it proc. some other things like
- Overpower increases MS/AS by [15-25]% for 4 sec (instead of on killing an elite)
- Overpower reduces CDs by [1-3] sec
- Overpower deals [50-100]% additional damage against Elites & Bosses
e.t.c.
BUT, in case if interested I think the bigger problem is related to itemizaiton early game (that’s where items should “tease” people enough to try builds and see what they like)
TLDR here’s a thread where I have summarized everything (more or less)