I played a human warlock from 1st to 15th level. It was a blast.
My favorite aspect was that I always had something to do. Since the invocations never run out, I didn't have to worry about conserving them for later, so I could take a productive action practically every round. If the enemy is buffed, voracious dispelling. If they're dumb bruisers, charm. If they outnumber the PCs, control the battlefield with chilling tentacles. If all else fails, ranged touch attack for direct damage.
Defensive strength is good too, especially with the Battle Caster feat. I got through RttTOEE with only one death, and that one from friendly fire. (I had lied about my alignment, telling the party I was Lawful instead of Chaotic, so the party cleric didn't bother to be careful with order's wrath.)
For noncombat Swiss Army magic, the warlock has two best friends. The first one is a collection of scrolls and wands. The second is the dead walk, because disposable zombies are great for trap-finding, chest-opening, and so much more.
Long story short, it's the perfect class for people like me who get bored when they have nothing useful to do, but not for people who want to use a different set of actions in every combat.