It's difficult to know what to say about this book without picking it apart, which would be a serious disservice to this story in particular. Reading Annihilation is submerging yourself into a sense of creeping dread. It's not the sort of horror that jumps out and says Boo. Instead it is the sort that makes you feel unsettled, like awaking from a disturbing dream. This is both the real strength of the book, and also the reason I can't say I completely enjoyed it. (Due to a history of anxiety I'm not especially fond of media that stirs up uneasiness.) Add in a dash of isolation, as the whole story takes place far removed from civilization, and the resulting mood ends up feeling similar to a mesh between Lovecraft and LOST.
This book is more of a character study, and a setting exploration, than a plot piece. You dive not only into the mysterious Area X, but also the mind of the POV character, the biologist. I really appreciated how lush the descriptions were with biological details - it not only fleshed out the setting, but it illustrated the way she sees the world. When you get deeper into the story it's hard to imagine a different perspective being anywhere near as effective. I should also mention that even though I was never quite certain how I personally felt about the biologist I did think she was a well formed, round, and convincing female character. And the prose was well formed and engaging.
When it comes down to it most of the things I have to say about this book are positive. It's a really strong addition to New Weird fiction. If delving into a story that taps into the uncertain and the bizarre, and can have multiple interpretations, then this might be your cup of tea.