Consider the following:
-A Warlock can do 3d6 damage to one target with his eldritch blast at Lv 5, all day long, from 60 ft away (longer if he has Eldritch Spear).
-A Wizard can do 5d6 damage to multiple targets with fireball at level 5, from 600 feet away, as many times as he can prepare it.

-At level 5, a Warlock has a grand total of 3 invocations, including blast shapes and eldritch essences, and it takes a feat to add an extra invocation.
-At level 5, a Wizard has 11 spells above Lv 0 plus all the Lv 0 spells, not including anything he may have learned from scrolls. It just takes him gold and some time to learn spells.

Even if the Warlock can do all day long, the Wizard is still better at its most "suboptimal" strategy, which is blasting, and can do other things on the side that will ultimately be much more helpful to the party.

The Warlock is further hampered by his rather pathetic skill point gain (since he's not intended to have high Int like the Wizard).

That said, I love the Warlock. He can really shine with UMD and wands, and is really quite flavorful.