As a DM, I find the warlocks annoying, but not because they're always zapping things. The invocations, while rarely powerful, can be extremely useful. Since many of them last 24 hours and the warlock can fire them off without real limit, many of the powers can be considered virtually permanent.
I played a warlock throughout Red Hand of Doom. While the character was almost never useless in combat, he was also almost never the star. He could be counted on for a fairly consistent damage output, but never really competed with the barbarian or the wizard.
He always had a few really nice tricks, but there were situations where he had little that would help at all. The biggest annoyances for the GM seemed to be the ability to spam Dispel Magic and to fly (slowly) all the time. (From other reports, spamming Shatter is also useful/annoying in many groups.)
Not overpowered; if anything underpowered.
All that said, I would actually recommend against the class for the player (not the GM). After about six months, I started to find it boring. You're only slightly more versatile in capabilities than a Fighter and because you can't survive in close combat, you end up hiding around the corner or up in a tree and sniping.
It's a good trick, but it's not an interesting trick.
YMMV.
As a one-shot bad guy, warlocks are woefully underpowered. A level 6 warlock is going to be doing 3d6 damage per eldritch blast. That is an average of 10.5 damage per hit, which requires a ranged touch attack to hit (not always easy at level 6 with a BAB of +4). Plus, the warlock is only hitting one opponent per round at that level.
A level 6 sorcerer is going to be able to cast at least 3 fireballs per day, (most likely 4 if they have a decent CHA) each doing 6d6 damage. That is an average of 21 points of damage per round. Yes, you can save for half and get the same 10.5 the warlock gets with no save - but, the fireball can affect many targets with its 20 foot radius area of effect.
After that, the sorcerer can crank off at least 5 scorching rays, each getting 2 rays at that level and doing 4d6 damage with no save. That is an average of 28 points of damage per round, if the sorcerer hits twice. (again, no given)
Then, the sorcerer can crank off at least 6 or 7 magic missiles. Each round, he would then do another 3d4+3 damage, or 10.5 per round.
So, the sorcerer has put out way more damage over 14-17 rounds of combat than a warlock of the same level. And, the times where his damage is equal, he can affect multiple targets (fireball with made save) or he has no miss chance (magic missile)

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.