I think what it does is take little used feats (and I mean very little used) and make them usable. It allows the spellcaster to use a metamagic feat without using a higher spellslot and can do it on command, but only once a day. Compare that with other feats like rapid shot or cleave or spell caster prodigy. Feats normally are always in effect and can happen many times a day.
Also, each one has the prerequisite of another metamagic feat, except for sudden quicken which has an ungoldly number of prerequisites, which it should, and can only be taken at very high levels.
I've never been much of a fan of metamagic feats because they are double limited. You have to memorize them in advance, not knowing what would be useful to maximize or enlarge, etc. (if you're using a wizard) and they take up higher level spell slots. AND you have to use a precious feat to do it. In 3 years I've seen them used only a couple of times.
I've begun to playtest them out in my offline game and so far so good. These sudden feats really seem like that extra card in the spellcaster's hip pocket they can play once a day to get them out of a jam or trash someone like a smite evil or such. And like those abilities it's easy to not use it because you're waiting for the perfect moment to unleash it.
With the prerequisite of another meta magic feat you also have to build up to it. You can't just start with one unless you are a human and get two feats. And if you're doing that then you're not taking feats like spell casting prodigy (which is huge), spell focus, combat casting, etc.
My thoughts on it so far.
