To be honest, Starfinder hasn't been very exciting to me. But this is hands down my favorite Starfinder book. Paizo really did an outstanding job with this one. Some things do need clarification(Mystic's Solar Weapon and proficiency, how Solar Flare works with the Soulfire fusion), but pretty much all of the options in the book are great.
The three new classes: Vanguard, Biohacker, and Witchwaper. All of them look very well done. As I usually lean toward martial characters, I'm digging the Vanguard. Haven't had a chance to play one yet, but looking forward to it. The design seems well suited to making a tough tanky character. This Vanguard has so many things it can do just at Level 1 with its Entropic powers. More faster, hit harder, and more. Many things that I think could eventually be incorporated into Solarians.
I really like how the Biohacker turned out. Being able to make a character that can shoot medicine into allies and poison into enemies with a hypodermic needle shooting sniper rifle is amazing. There are a good number of new weapons in the book, many of which are Injection weapons.
Don't know much about the Witchwarper, I never really looked to closely about how it worked in the playtest and I haven't had a chance to look at it now. I hear good things about it and most people seem very pleased with the final result.
I think shields will be well recieved as some classes have been needing some kind of boost to their AC's.
Love the alternate options presented for Races & Classes. So much more flexibility and variety for Starfinder with this material. Unarmed Operatives, Ascetic(or Aesthetic)Monk or Raging Barbarian Soldiers, Solar Weapon wielding Mystics, Mechanics that can build their own experimental Ironman suits, Magic using Envoys, and Shield wielding Solarians are just a few of the things given inside.
The Race options really help to bring Core and Legacy races back in the game compared to some of the newer races. More options for ability score bonuses and penalties, so you'll most likely be able to make that Race/Class combo you've been dreaming of work and work WELL!