For me, and this is just my opinion, the voice acting is absolutely superb. The narrator in particular is incredibly well done. That's a huge amount of lines that poor man had to read, but still did a great job and didn't lose his 'enthusiasm' to make it captivating.
One of the problems with voice acting, is that if a mistake is made and not caught, it can pull you out of the immersion. For example, when you click on a globe in the game, the narrator says 'And you thought
Rivellon was flat'. However, he puts the emphasis on 'Rivellon', making it seem that he's comparing how flat Rivellon is versus some other place. He should have put the emphasis on 'flat', emphasizing that Rivellon isn't flat, despite what people believe.
Little mistakes like that can creep in and if not kill the immersion, it will at least lessen it.
I don't really mind the voice acting overall (though I'd be perfectly fine with just text), but minor niggles like that do tend to distract from the game.