According to the feat's wording, it allows you to either use a grappling hook as a light piercing weapon or as a two-handed weapon with a reach of 15 ft when used with a rope or chain, if your character is proficient with whips. Above all, it can be used to perform trip, drag and reposition combat maneuvers.
According to the rules about reach and threatened areas, large humanoids typically have double the reach of medium and small creatures with all melee weapons, including the particular case of reach weapons. Does this mean that a large creature with Hook Fighter can get a total reach of 30 ft (and that looks ridiculously strong considering the possible combat maneuvers that can be used), or does it work differently ?
Typically being the key word. Some large creatures only have reach of 5 (example that doesn't super apply, as to my knowledge they can't wield a whip but Dire Bats). Plus consider the large creature in question needs to be proficient with the whip for the reach, which is an exotic weapon. So with that in mind, most large creatures, even with class levels, would still need to sink 2 feats towards it, meaning min level 1 (with a fighter due to bonus feats, and having the BAB to take them), but most would be waiting till at least 3.
This denies the creature other feats, such as improved maneuvers and other such things. It would have it's size bonus, and base. No toughness or other such thing to round it out, etc. It's scary, but doesn't provoke any squares while using the hook in whip mode, so even with 30ft reach it's on par with players of an equivalent level, as they can directly rush it and take the ranged maneuvers advantage away. On the player end, short of being a fighter they're devoting a lot of resources to this, plus playing a large creature, many of which have adjusted CR, which would make them feel significantly weaker for what they're getting (short of other racial abilities).
you are correct you get 30' reach and can attack any square within that 30' area.
The main drawback I see is that you don't threaten any squares when using it as a reach weapon. So you can not take any AoO, and you do not provide flanks for any allies.
and actually the inner 10' is fine because the feat calls out that you can attack anywhere within the 15' reach, normal reach weapons do not have that verbiage so you are unable to attack inside the actual reach value.
You do not threaten any squares -with the whip- You still threaten normally if you have IU or are holding another melee weapon in the other hand (which is to say 10', not 30', and you can only make AoO with the threatening weapon(s)). I personally prefer a Medium character with a whip, lunge, and a cleave feat tree (less of a target).