It does a decent job for the most part of providing rules for playing a modern era pathfinder campaign. While the talent system creates almost as much flexibility as point buy systems, and is a lot of fun, there are some serious problems with the Modern Hero class. This is a problem since Modern Hero Class is the bulk of the book, and is supposed to be a viable class even with the traditionals running around.
Day Job & Perform: Incorporates the day job rolls from Pathfinder Society, but requires them to be profession only. PFS ends at twelfth level, whereas this is supposed to go up to twentieth so the table really should have been extended. Moves all the creative stuff ot Perform, which seems odd. Doesn't provide any reference for using Perform or Craft in the age of the internet. Is the internet just a prosperous city? If you take perform writing, how would you sell your stuff? Presumably the day job check could cover this, but using Perform skills isn't allowed for that by RAW.
Modern Hero (Human Only): Only humans can take it as a favorite class, because the other races hold onto the old ways. So if you stop being human, you lose the ability to have a favored class at all. Similarly a half-elf/half-orc/tiefling/aasimar who is raised purely by humans is holding onto the old ways of people they've never even met? Then there's the question of variant or mutated humans counting as human enough. Not a rule that makes a lot of sense, even if it is just a favored class bonus.
Modern Hero(Broken paths): Put bluntly three of the six paths listed serve no purpose. Strong and Tough hero are identical, except that tough hero gets a bigger hp die. Smart, Dedicated, and Charming all get the same saves, attacks etc. But smart gets 2 more skill points per level. So Strong, Dedicated, and Charming have no reason to exist mechanically. Only difference between Fast and Smart is which save is their one good one.
There isn't a limitation on archetypes or talents by path (sort of with mystic. see below) so they won't be getting used. I think cribbing the CoC d20 assortment might have worked out better, and will probably try it in the future.
Modern Hero (Multiclassing is mandatory): Each Modern Hero gets 6 class skills, and can improve the number by two each time they waste a talent. This means not only does a Smart hero have more skill points than skills, they have fewer class skills than a fighter. Kind defeats the greater versatility due to modern living they talk about. So expect most skill oriented heroes (Fast or Smart) to dip into Aristocrat, Bard, Wizard or one of the other classes with a massive skill list.
Almost every melee modern is going to dip into Fighter, and vice versa. Why? Because only one archetype gets all martial weapons, and none of them get armor. Much faster to boost Fort Save, get a massive increase in weapons armor and shield along with an extra feat. Fighters will dip because Extra Talent is a feat and they can boost BAB, AC or Melee damage by +1 if they take it. Yeah most combat classes will do, but the effect here is extra nice.
I personally don't have much of a problem with multiclassing, but pathfinder seems to push the philosophy that a well designed class shouldn't require it to be playable.
Modern Hero (The Mystic): Oh God, the mystic. Suggests that the character's spellcasting attribute should be the same as the chosen path, but never actually tells you that you have to pick one of the back three for your path. Also the only archetype that limits paths, and I think that actually contradicts other rules.Can cast Arcane, Divine, or both. Might have a spell failure chance with arcane, it's never mentioned. Spontaneous caster that maxes out at four slots per level vice six. The only full progression caster whose cantrips aren't at will usage. Gains special powers by taking Oracle curses, which is actually kind of cool. Requires two training at a minimum to accomplish any spellcasting, since you initially get the progression and universal spells only, so plan accordingly. Other than the cantrips, so far so good.
Two fastest ways to add to your spell list are to take a curse then use advanced training for either the Oracle or Witch spell list, or pick an arcane school as a regular training. Arcane school lets you pick both arcane and divine spells of that school. You'll have to figure out how that interacts with spell completion and spell trigger items. You can do all three if you want.
You can add a domain for a training/feat, including the domain abilities, which seems overpowered. You'd think the domain ability would require another investment. Says you get bonus spells, but not whether that's bonus spells known or the cleric domain slot. You can take this multiple times, but there's no mention of how they interact.
Further, picking up a mystery is available as an advanced training. Despite listing how saves for the revelations work in the training description, it looks like you have to take an additional advanced training (and curse number 3!) to pick up a single revelation. The other advanced training is to take a curse and gain a hex which is even dumber. Really seems overpriced when the cost of a mystery is compared to a domain.
There's a few more things for the mystic, and the idea isn't a bad one. However, the actual rules are a badly written mess and most GMs are going to rewrite it prior to use.
If you're doing a modern, low magic campaign without the core classes the modern hero class is actually really neat. However, if you're using it as intended expect a lot of multi classing or a high probability of players sticking with the core/base classes beyond a dip. Retrofitting the archetypes to core classes has a section at the back of the book, and isn't a bad idea.