The problem with "Instant OFF GCD" abilities makes combat feel less rewarding.

[QuantumDelta]

-snip-

I'm very aware of the mechanics, however when it has an effective casting time of /longer/ than Cure III's actual cast time, it makes you a little curious as to what the actual point is.
Heals like Renew in other games are generally seen as mana efficient ways of minimising overheal, I've found it most useful chucking it on targets that have taken periodic (normally failing to avoid avoidable) damage, rather than on the actual tank, and letting it sort their HP out.
It is very slightly more mana efficient than Cure III, but for actual 'usefulness', especially with it's 2.48 second effective cast time....not great.
Like I said, I pretty much only dump it on the tank when the tank is about to run in or is on HP where it's no issue if I'm on GCD for a two and a half decades.