I've been extremely impressed with GPT-4 so far, generating a campaign for my kids. The encounters it created were very well fleshed out (monsters, minions, stats and abilities for all, including very flavourful lair actions for bosses). It's given me NPCs and sample dialogue for each, locations, and a central storyline that is well-threaded throughout. I was genuinely astonished even at its first attempt. I don't know if it could stand up on its own as an interactive dungeon master, but as a tool I am loving it.