Balance is a problematic term. People tend to think that something that gives a huge advantage, but includes a huge disadvantage, is fine because there is a balance. In truth, things that work along this principle usually result in two problems: they are excessively strong in one regard and excessively weak in another.
These feats allow a character to toss in a metamagic effect 1/day without increasing the caster level. During that 1 casting, the spell is more powerful than it should be. This results in the character being able to do things that were not intended to be in the hands of a character yet. For instance, a suddenly maximized fireball cast by a 5th level caster (30 points of damage) will have a dramatic effect on that one battle ... an effect far stronger than was intended to be in the hands of characters of that level.
On the other hand, during the rest of the encounters that the PC faces, s/he will be at a disadvantage because s/he will effectively have 1 less feat than they are supposed to possess.
The end result? The DM has trouble balancing combats. The DM has to plan around that suddenly maximized fireball. A sudden maximized fireball can wipe out an entire encounter that was meant to be a challenge. If the DM anticipates the use of the fireball and makes the enemies tougher, he may end up with a TPK if the caster unexpectedly uses/wastes the suddenly empowered feat on something else earlier and doesn't have it when the DM thought that they would have it.
If the DM doesn't plan around the presence of the feat, his big encounters can be pretty boring. Few things ruin a good session as much as a BBEG that dies too easily.
Randomness in the game keeps it interesting. Too much randomness in the game makes it chaotic and wild. That results in more encounters that are too easy or too hard. Feats like this one are a big source of randomness in role playing.
Is the feat balanced? Perhaps. But I do not think it is good for the game. Things that grant too much power for a given character level, even in a limited situation, are bad for the game. Things that are focused into narrow pidgeon holes instead of being spread out over many encounters are also bad. These feats are bad for both of these reasons.
