This one is a little hard to rate, because the niche it fits is a strange beast indeed. There are a comparatively small number of settings using Pathfinder or some variation to simulate non-swords and sorcery type worlds, at least in comparison to the VAST number of supplements that support a default high magic medieval fantasy setting. This one targets an even more tight niche: superheroes, a realm that 3.x and Pathfinder have always struggled with handling. So, bottom line, there's nothing to compare it with out there.
However...with that being said, this book is wildly creative. It provides statistics for a very wide variety of adversaries, from anti-mutant activist humans with guns all the way to godlings and cosmic threats. There's something for damn near everyone in here, and despite having a focus on comic tropes from the 90s there's more than enough love shown to other comic eras that you'll get a smile out of the references (hell, one list of minions outright arms them with Kirby weapons).
There's have you evern wanted to throw your PCs up against Dr Manhattan? Or Captain Atom, or Firestorm? Want a quick Ozymandias style mastermind served by legions of self-destructing ninjas, engaged in a war with an evil Superman analogue and his codenamed, tights-clad superhuman servants? These are all options, and there's plenty more. While focused for a modern/superheroic/sci-fi campaign, any GM worth their salt can simply file off the serial numbers and reskin them for their own game. Further, there's a mix of humanoids (and others) with prebuilt class levels alternated with 'monster' statted superhumans (see the Codenames referenced above), which in turn lend themselves to Simple Class Templates or the like. And, on top of all that, there's multiple variants of many different enemies, sometimes reskinning one stat block to function as a different enemy faction altogether. And that's not getting into the templates in the Appendix. Basically, if you're running a game that has modern or high-tech influences, you'll find something useful in this book for you.
Oh, monster statistics for vehicles. Want to fight an aircraft carrier? Have a raging barbarian leap from a skyscraper onto an attack helicopter? Got you covered.
And on top of all that, there are a handful of enemies that play with the conventions of the genre in interesting ways. The Eugenecist Demons, for instance, are evil outsiders with a specific focus on mutants. That's an amazing idea that exploits the intersection of superheroics and fantasy tropes in a great way. After all, if you had Abyssal legions or the Daemonic servants of the Four Horsemen in a superhero world, there's absolutely no reason they wouldn't have specialists who focused on specific power types.
Some of the enemies have problematic abilities. The Fallen Paragon (evil Superman analogue, though the flavor text is really more Supreme from 90s Image comics than Supes) is the standout in my mind - it has an ability that allows it plot-level strength to lift/crush/destroy whatever drama demands it be able to do, which is fine in and of itself. But it also specifies that it doesn't fail combat maneuver checks. That's a HUGE problem, because it means the thing can simply swat weapons out of your PCs hands nonstop. I would probably modify that to 'doesn't fail defensive combat maneuver checks', so it's well-nigh impossible to knock something out of its hand or trip it yet it 'merely' has a huge Strength and Dexterity bonus to its already significant CMB. Similarly, some cyber-implants for various enemies allow outright immunity to a kinetic damage type (bludgeoning/piercing/slashing, though never more than one), which to me is a bridge too far outside of slimes or truly amorphous opposition - I'd probably houserule those into DR/15 against their particular type, which would mean average folks couldn't hurt them but PCs would be able to breach.
A lot of the equipment and cyberware references Technology Unleashed (another sourcebook from Skortched 'Urf). You don't necessarily need that book, as the stat blocks give the pertinent information to use the gear (kudos, Mr. Fields), but some references may lose you or not make sense otherwise. Also, these are built more on a D20 Modern/3.x framework as opposed to the Pathfinder Technology Guide setup, so
OTHER "YOUR MILEAGE MAY VARY" COMMENTS:
Mr. Fields is definitely more conversant with 3.x/D20 Modern than Pathfinder, as some of the enemies here are upconverts from those sources. I'm the same way - just too much 3.x material on my shelves to abandon it. But it does lead to some design decisions that Paizo wouldn't do. Again, not a problem per se - things can be altered on the fly without too much trouble.
Also, the section regarding using Millitary Installations as an enemy is a little funky. I LOVE the idea of abstracting built-in defenses and hordes of 1st level grunts as attacks and spell-likes, kind of like a giant construct with a built-in swarm encounter and a lot of special abilities, but it feels weird on some level. Might be better served as a sort of event-type encounter like the Manifestations of the Great Old Ones are in Sandy Petersen's Cthulhu Mythos are. But that's an entirely personal observation.
It's creative, it's more than a little gonzo, and it should give you ideas for days. This is definitely recommended if you even have the slightest use for enemies above the basic tech level of a medieval fantasy world. Heartily recommended.