Indeed, pretty good first few hours for a civ release!
I did have some recurring crashes though in longer game (after turn 300ish with 6 players) but at least there is autosave.
Otherwise performance it's really good, it felt better than civ5 in late game. The game is still responsive while it's the IA turn, that really makes a huge difference.
As for gameplay it's pretty much all that was expected from pre-release info.
Maybe playing wide is a bit too OP though if not completely required.
Btw you can somewhat stack units again, there's also a merge option for units of the same type.
Overall a good game, spiritually not that far from 5. If you are short on budget it's probably still worth playing 5 and waiting for 6 sales/expansions.
There are definitely things that could and will be improved over time such as mentioned, IA (I still need to test the top difficulties but in prince/king it was really too easy), UI (in general I like it except that some important features are too hard to find), some buildings/districts that feel much less useful than others, ...
Keep in mind this is also a quick review after only a game.
Both in top 10 now that's something. And pretty much the first time the 5 is so slow since its release!