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I'm 100% sure I'll buy this game but I don't want to spoil the story. Is it a more open ended game with a mainish story and lots of side quests to explore or does it have more of a focus on main story? Should I wait for 1.0 release to experience the finished product?

I'd appreciate thoughts.
Hi Thoogah. Not a great expert on the game but just my 2 cents. From what I've read, Act 1 is now playable, and act 2 and 3 will be added in later patches. I believe sidequests may be added intermittently. So I think you can safely play the game now, and do the main quest until the end of Act 1. And then in a couple of months, reload the same savegame and play the next acts when they are released?
Easy answer: Yes.
Wait till it is finished. Otherwise you might get to a point were you can progress the way you want because that path is not done yet.
It depends on what you want from the game at this point. If you want a complete story that's polished with few-to-no bugs, then absolutely, wait and hope for the best as far as bugs go.

I got this early because I was excited to see an early, working version of the story, and how it might change in future iterations. Even if it's just little tweaks here and there to questlines, I'll be able to look back from the final release and remember how it once was in the early stages. I'll say now, however, that so far most of the story elements for the self-contained side quests in Act I seem complete, with little need to altar the story beats. But we'll see.
I think that if you get an early access game and hope to get a representative complete play-through by pausing for new content, then you are setting yourself up for disappointment. There are lots of ways that saves can be rendered unusable (one of which is that the developers' release version of the game is incompatible with saves from EA - it happens...). Early access means the game is not ready. If you want the complete experience then wait till a couple of months after release to play it.