Snapdragyn
Explorer
3. The absolute key to making an effective warlock is invocation and spell selection. If you don't plan these out carefully, you can flat out miss what's basically a core class feature. Hex, for example, adds to your damage for pretty much a whole encounter, since it transfers from one target to the next and doesn't have a saving throw. And if you DON'T take Agonizing Blast or Thirsting Blade, you're really screwing yourself. But if you plan this stuff out carefully, you can have a quite effective character, IMHO.
(Bolding added)
Or you're taking Misty Visions and planning a crowd-control oriented Warlock who can pop off slot-free illusions at will. Seems pretty decent to me. YMMV.