There's already a thread about it around here somewhere....
That said, I play multiplayer once a week. 2-5 humans, and a dozen or so computers. I've got 418 hours in, probably more multiplayer than single player. The AI isn't perfect, no AI is. But I've always found it superior to most other strategy games. Far less brain dead proposals from the AI. It's pretty rare for a weaker AI player to antagonize a stronger player. Stronger players generally get tired of weaker players and either subjugate them or wipe them out entirely. Allied players actually help out in wars.
From a technical stand point, late game stutters/stalls is greatly improved over the early patches, although yesterdays patch seems to have brought a little of it back. DLC wise (IMO):
Nova/Galaxy Upgrades (on Steam): No.
Plantoids: No.
Leviathans: Eh. I could go either way on this one. It's not adding a lot, but the content it adds is fun and a nice diversion during the boring mid-game. Currently 33% off on Steam, so at under $7 I'd probably hop on that.
Utopia: Yes. It's playable without, but the megastructures and perks add a lot, especially late game. If you're not sure you'll like the game, I'd hold off as if you don't like it without Utopia, you won't like it with. It's not on sale or anything so there's no rush.
There's a handful of mods available on the Steam Workshop that I highly recommend as well. Especially AlphaMod.
