For fans of Spirit Island like me, it feels essentially impossible to have too much of a good thing. We really love this game, and this expansion brings more of what we love.
The new spirits that we have played so far are interesting, as are the new aspects for the older spirits. The new Incarna mechanic is interesting, providing the feeling of having a physical presence on the island for the spirits/aspects that use it, without feeling like it puts those who do not at a disadvantage.
If, like me, you can't get enough of this game, I'm sure you will enjoy this expansion. If Spirit Island isn't for you, or if you find the base game is pushing the limits of your personal preference in terms of complexity, this expansion isn't going to change that so you should probably give it a miss.