This is one of the coolest creature books I've read in a very long time. With few exceptions, creatures in this book have expansive descriptions. Life processes, society, desires, personality tendencies, and many more flavor articles are common. (Dragons and elementals are notable exceptions, with very short flavor descriptions by comparison)
Better still, around half of the monster have a player option. ~20% of the creatures are playable races, but even non-playable monster often have a player option. A magic item, a series of weapons or armor, a consumable, something. This makes the book an attractive one to look at for all player, not just GMs. I'm doubtful the player options are reason enough to buy the book on their own, but I'm very glad that they exist.
Importantly, every creature in the book feels like it should be here. You have the "generic needs to be here monsters" like the reptoid, the swarm, Oma (read, space whales) and a few updates to old classics (Ryphorians = Triaxians). Everything feels like it fits with the Starfinder model. It's refreshing to like every monster in a bestiary book, again.
The monster creation rules are easy enough to use. In short: Pick if your creature is a caster, a combatant, or an expert. Pick a CR. Look at the appropriate tables for HP, KAC and EAC, attack bonus, damage, and # of special abilities. Pick a few abilities that work with your concept and you have a functional statblock. You can make it more complex if you want to, but it doesn't have to be.
Plus, grafts. Grafts are a great GM tool. Sad that there's no demons in the book? Take any appropriate CR statblock and slap the outsider and demon grafts onto it. Now it's a demon.
The downside to this easy to use monster creation system is that you're expected to use it. Going back to my earlier comment regarding dragons and elementals: their flavor text is small because they have multiple grafts that give you the special abilities for the different kinds of monsters (fire, cold, air, earth elementals and the different colors of dragons). This is fine for elementals, which have a different statblock for each CR from 1/3-9, with only the special abilities being different.
Dragons, on the other hand REQUIRE you to do the prep-work in advanced. There is only 1 dragon statblock, and it's a CR 11. If you want a dragon of any other CR, you pretty much have to make it from scratch using the tables in appendix 1 and applying the right grafts. I liked the bestiaries having three statblocks per dragon variety.
Generally if I wanted the PCs to fight a dragon I could grab any bestiary and have at least 1 CR appropriate dragon for them without much work.
My biggest complaint about the book: It's the smallest Paizo hardcover I own, by a lot. It is 160 pages long. The next smallest I could find (that I own) was 254: Advanced Class Guide, Villain Codex, and Pathfinder Unchained.
The Alien Archive costs the same as Bestiaries 3 and 4. Each of those has over 300 according to the Paizo website descriptions for them. The Alien Archive has 95 stat blocks, which includes 3 ready-for-combat starship encounters. This is small by comparison. It's cheaper than 2/3 of Bestiaries, sure, but I'd have happily paid another $5 for an equally large book. When this is the ONLY Paizo monster book for the game, that's a big problem for me.
The content is great, don't get me wrong, I was just hoping for more of it, considering what I paid, and I will be more wary of Starfinder products in the future because of it.
TL;DR: Great Book. 5 star content. Small size knocks off a star.
