New class stuff is awesome, new equipment is always nice, and the vehicle section I will find the most useful as the GM.
I found the Mech building rules to be an annoying convoluted mess, but the rest of the book is solid. I plan on using NPC building and Custom Vehicle building rules and an alternative system since the end result is mathematically close enough with less hassle, especially since in the VTT I use (Fantasy Grounds) I can't offload mech creation to the Players between session due to technical limitations of the system.
I'm still absorbing it, but beyond lots of cool creatures it adds quite a few "base" creatures and template grafts. Alien Archive 1's charts are great if you have time, but if you need some quick stats mid session for, say, a void dragon t-rex, you have one in a minute or less. I really like the improvements to the appendix and wish that Alien Archive's was that good - for example, it is nice to have a list of playable races and what pages they are found on. Starship-scale foes are few, but it's nice to be able to kill the giant space worm and then make a ship out of it's corpse.
The variety is great, and overall the book is up to usual Paizo quality (as in, pretty darn good). However, I am giving this book 4 stars instead of 5 because of the massive ammount of new categoris of fiends: Asuras, Demodands, Divs, Kytons, Oni, Rakshasa - all new categories of fiends that join the ranks of devils, demons, and daemons from previous books. It is a bit overwhelming to have that many new types of fiends in a single book.
As a DM, I see a lot of useful things I can use. As a player, it seems a little overwhelming. Overall, I like what I see. Some things look a little strong, some things look a little weak, and most of it looks crazy fun (and I am a fan of crazy fun). There are even some things in here that I've actually discussed with other players, both on and off line.
I give this product 4 stars for one particular reason: GMs are going to have to make some serious decisions on what to allow and disallow. This is great for custom tailoring campaigns, but it means that I (as GM) have to be a little more controlling than I like to be.
