I've been debating getting dishonored for a little while now, and i just have a few questions that are kind deal breakers/makers for me.
Is it a more open world setting like bethesda usually does for the fallouts and TES? Or more like Brink?..
What is the customization like? Is there only a set way to be the assassin or just one way?
How is the story/writing for it?
Thank you in advance

It's not really an open world game. It's just that each level is widely spread out to where you can play it in many different ways.
You can experiment with your whole arsenal of abilities and weapons, or can just play it safe by sneaking around enemies to complete the objective.

The only customization in the game is your weapons and character mask being upgradable.

As far as the story goes, it rarely go into detail at all, with an unsastisfying ending. The game is pretty short itself too.
Playing it atleast 2+ times, reading alot of books, and doing all the side missions will make it alot more fun.

I think you should give it a chance. Despite the downsides from it, the gameplay is what really made this game.
If you're iffy about buying, the least you could do is rent.

Bethesda did not make this game. Let's start there.

Customization goes as far as allowing you to choose your equipment upgrades and powers. The character himself is set in stone, which is something I prefer for this specific game. You can, however, utilize the tools given to you in any number of different ways, which is one of the highlights of the gameplay.

I loved the story/writing, unlike the person above. The world created is very believable.

The game isn't really short if you take the time to explore and do things secondary to the main story (or just do things in new, really badass ways ), and replayability is also very, very high. I think I completed the game...nine times?

So yeah, I'd highly recommend it. Though everyone's tastes are different, it was my personal favorite game of 2012.

As mentioned above, Bethesda did not create this game, Arkane did.

Dishonored isn't an open-world game. The levels have various routes through them, but they're not completely open-ended like the levels in, say, Thief.

As far as customization goes, you can choose various powers and weapons which can allow you to perform various assassination methods, although the game is lacking in non-lethal options.

Personally, I find the storytelling elements to be a mixed bag. The dialogue and voice acting is wooden and the overarching story isn't very strong. On the other hand, the environmental storytelling and the world itself are really well done. The ending isn't that great either. When I first played it, it took me about 18-20 hours to complete, although the level design means there's plenty to do in replays.

I wouldn't go as far as to call it the best game of 2012, but you should definitely give it a try.