I've been playing around with GM mode for a few days now and although it's highly limited, I still love it. I'll echo what's been said so far in that it's very much just putting a visual element to traditional pnp rpgs rather than letting you rebuild Divinity 2 with your own story (I gather that's what the editor's for).
I think people have brought up some very valid concerns with the GM Mode regarding functionality that would greatly benefit GMs (like being able to select groups of NPCs, RTS style. Enforcing camera position for certain scenes. Adding some more interactive objects for the purpose of animation or scripting. etc), but quite honestly I love the mode for what it provides.
Pnp RPG prep takes a goddamn age. I've only ever really GM'd Shadowrun, but the amount of work that goes into making a believable world is... exhausting. Saying that, we do it because we love it.
I tend to build my own library of NPCs/reward chests/enemies/special items and weapons in advance. Things that I know can just be dumped down on the fly. That way you don't really need to panic too much about building up every single aspect. The one thing GM mode is really good for is doing things on the fly.
If anyone has any hints and tips for making things run more fluidly, I'm all ears
