I vastly prefer the Warlock to the Wizard in nearly any setting, simply because they aren't godlike from start to finish.
They are strong, yes, and they have staying power. These are good traits to have in a base class. But they can't even begin to spit at a Batman Wizard, or typical CoDzilla, or even an Artificer or Archivist.
One cheezy trick to do is to dip two levels of Chameleon for the shifting feat. This plus the level 12 Warlock ability means they can make literally any non-artifact item in the game with trivial effort (although it still takes an expenditure of xp and gold).
Hellfire Warlock is... meh. Rawr, it can do damage, but not nearly as much as even a standard Charger build (Power Attack + Leap Attack + Shock Trooper + Pounce). Even with Glaive, it doesn't begin to compare.
While an interesting concept, I don't approve of getting an entirely new 'sphere' every level. In fact, I'm not sure if 12 spheres (new sphere every time a current Lock gets a new invocation) is balanced. That's a LOT of stuff to do, making them exceedingly flexible, and some of those powers on those lists are pretty broken.
I've tried to build some base classes to replace the druid/cleric/wixard/sorcerer/bard based on the invocation system. Cleric and Wizard have one invocation per level, but 1/2 BAB. Druid and Bard have standard lock invocation advancement at 3/4 BAB. None of them get broken stuff like Divine Power.