Your not taking quicken eldritch blast, you are taking quick spell-like ability, which applies to ALL of you spell-like abilities one, if you take it twice, it applies twice, to all of them, of which, invocations and EB are both spell-like abilities. So for one feat, you are basically getting sudden meta-magic twice.
Anyway, no, I admit I haven't played a Warlock, but I don't have to play every class to make comments on them, and niether do you.
IMO, any class that can use an ability unlimited nubmer of times a day is just wrong. Any class that can use that same ability unlimited times a day while appling effects directly to the ability, making it more powerful, even worse.
I haven't mad a warlock to try and break it, but with the unlimited ability of the invocations and the EB and 1,000s of feats to choose from, I'm sure if I put in five man hours I could find a way to break it. I'm sure others already have.
Anyway, at fist glance, I think a sorcerer is worse, but that is just me and the fact that I have played and broken a sorcerer and I haven't tried a warlock.