What's an Invocation worth?

[Submortimer]

Hey all, I have a rather simple question for you: exactly what is an invocation worth?

What I mean: While trying to homebrew fantastic, new, and fun Invocations for everyone's favorite devil-worshiper, the Warlock, I realized that aa few of the things I was trying to do either grabbed from class abilities, from feats, from parts of feats, and from spells. What I'm trying to figure out is waht should an invocation slot allow you to get? What is too powerful? IS anything too powerful, given the appropriate level pre-requisite?

Thanks for the input!

[pwykersotz]

My opinion is that it's variable. But I would use guidelines:

Reference the Spell Creation and Modifying a Class sections of the DMG. Good stuff in there.
Don't duplicate anything feats give if possible. Players can already use those options (unless you're disallowing feats).

Given that the two DMG sections I mentioned include so much, those are really the basics. And yes, given the appropriate level prereq and recharge time, just about anything within the scope of the class goes in my book. The invocations requiring 15th level should serve as good top-end limiters.