So.. even before buying DAI I was as excited as you, guys, about classes and, particulary, about Rift Mage. Mostly because of IMPACT detonator and almost infinite mana.
Recently got my hands on it, and immideately noticed something is wrong. Couldn't figure what exactly for a while. Did some tests, found 2 ridiculous things, one bad, one good, just like that old joke.
1, the bad one: Stonefist - it says it does detonate things. But in fact it doesn't. Or well, it does, but it's very weird and complicated. I'll explain.
As far as you probably know, IMPACT detonator can cause 4 types of detonations - Shatter, Discharge, Rupture and that one weak Stun+IMPACT. So, apparently, Shatter and Rupture don't work with Stonefist AT ALL. Discharge - only AOE part (no damage on detonated target - example screenshot). Stun+Impact works as you would think it works - Stonefist + mediocre damage without any special effects.
I really hope it's a bug and will be fixed soon.
2, the good one: Shocked+Weaken wombo-combo. I'm not sure if it's a bug or intended - though, it's not stated in any in-game description or tips and I couldn't even find it on the allmighty internet. I started noticing it for quite a while - usually it happend after Magic Barrage. I have both of Varric's sleeps on manual control, so I was like DAFUQ, what is that spooky sh*t??! (As I later realized that was Cass 5% passive, since I was not specced yet).
But when I DID - Chain Lightning + Pull of the Abyss - well, you can imagine the effect. Half of the freakin map asleep. Especially funny, when followed by AOE eldritch detonator, like Spectral Blade or (what I use on Dorian) - Dispel (example screenshot with imp. Veilstrike).
I really hope it's not bug and won't be nerfed soon 
Hope that helps. Have a good day, fellow Inquisitors 