I already said, that the warlock is an entirely different class than any other. That mean, while you can somehow categorize the other classes into some generic groups (like fighter, paladin, barbarian as the front-liners, ranger and rogue as the skirmishers/artillery, bard, cleric and druid as the support/healer and sorcerer and wizard as the arcane support/blasting), the warlock might be everything and none to it.
The basic of this view follows the fact, that the warlock uses a lot of unique mechanism, while other classes has some very basic commonalities, like Extra Attack for the weapon based, spellcasting trait, etc. So this means, that despite all differences, you can get a bit of insight what these class are supposed to do.
The warlock is supposed to be a main-caster in a way, but lacks spellcasting. He uses Pact Magic, which is quite different. He isn’t as squishy as a standard wizard/sorcerer, but have the basic defense capabilities of a rogue. And with the Invocation mechanism, he can be pretty well customized on a basic level (means there will never be need for a new kind of subclasses, which would replace standard features like Pathfinder archetypes do), be it in focusing on damage, utility, spellpower or other departments.
And of course he has practically 2 subclasses, one pact which would be the real subclass and the pact boon, which doesn’t have anything to do with the pact in the first place, but will affect many builds as much if not more than the choice of fey, fiend or great old.
This makes the options of customization fourfold: Known spells, Invocations, subclass, Pact Boon. A fey warlock with the pact blade will feel much different than one with the pact tome, since the focused went from enhanced weapon damage (and the utility to use the weapon you want unless you got another item bounded) to utility-magic focused type, even though both excels in mind-affecting and tricky spells. And if you would change to the pact of the fiend instead, you got your weapon plus a bunch of offensive potential.
So even when writing my overview I just get the feeling, I can’t really pin down this class as good as others. Even though the cleric is problematic, since his domain will give him a huge jump and change the class focus for a great mileage, the warlock is just more complicated. And if you get into feats and other non-class dependent customization, you can do about anything with this class, even though it might not excel in it. But if a warlock would be a better fighter than the fighter, I would definitely complain about it.
But for most people who are looking forward or have already fun playing a warlock, this variety might be exactly the reason why to. Personally, I’d like to play a fey pact blade warlock, combining a charming personality, swashbuckling and a bit of useful spells, even consider to not take Eldritch Blast as a cantrip or even if I do, to ignore the Agonizing Blast Invocation, since there are so many other nice ones there.