FormerlyHemlock
Hero
Nah. A class that can negate enemy spells is bad enough. A class that can negate enemy spells, then use their spells to power your metamagic... yikes.
That's exactly what a Staff of the Magi does. (Or really it's about 10% of what a Staff of the Magi does.)
I'm interested to hear how it would affect your playstyle. Would you play an Instinctual Sorcerer preferentially in hopes of getting the 14th level Magic Eater ability? Would it prevent you from dipping Warlock until after you'd already acquired it? Would you rather be a magic eating sorcerer than a luck bending Paladin/Wild Mage? If an enemy mage wraps you up in a Wall of Force and renders you helpless, are you still going to feel that your investment in Magic Eater was worthwhile?
If you want to discuss balance, put on your powergamer hat and let's discuss implications. My powergamer instincts say it's not that exciting. You're still going to have to learn Counterspell anyway; but every once in a while you may gain some free sorcery points. It's more "cool" than powerful. (Inspired by Thorned Namshiel BTW.)