A solid outing, the Kith of the Harpy Queen is primarily a presentation of several "themed" harpies - the "giant" harpy, the small, flock-focused harpy, the demonic harpy, etc. as a monster supplement. It also includes several harpy-themed magical items and spells.
It's always difficult to assess a supplement like this. The Glory Harpy's CR seems off base (it has not just one but three spell-like effects which are among the most overpowered effects for their level, and can take one of the harpy feats listed to multiply their usefulness), but that's true of the monsters in the base game too. I can't call it great because it doesn't rise above the bad judgment calls of Pathfinder/D&D3 with respect to monster evaluation (or spell evaluation for that matter), but perhaps I can't rightfully critique it on that ground either.
I should say I very much appreciate the pagination of the monsters. Ever since the days of the three-ring-binder Monster Manual for AD&D, I've been very attentive to how I can assemble monsters and other information for the campaign. Just print the page you want and you're good to go. (The same advantage isn't there for the spells and magic items.)
I do love harpies, so perhaps I'm giving this one an extra star just because they're such great monsters, but there's still a lot to do on the subject - a discussion of the mythological harpies and their roles, roles for different types of harpies in different types of campaigns or adventures, at least some attention to the gender politics of the harpy legends and how they might be used for political expression in your game. These types of analyses would be my suggestion for ways to improve the supplement.
Overall, however, the Kith of the Harpy Queen is a solid, straightforward, simple monster supplement that carries with it some attention to other factors of the D&D3/Pathfinder era which are relevant to players: spell choice and character building. It's worth a look.