There are a lot of bad things to say about foil action, but one thing it has going for it is that it is genuinely game changing. The ability to shut down a single opponent makes some kinds of encounters obsolete. A DM just can't throw a single boss monster against an iconic group of four adventurers and expect it to be a challenge. This is not a bad thing - around 9th level there are all kinds of abilities that fundamentally change how the game plays (raise dead, teleport, contact other plane...). Saying that foil action is a game changer doesn't mean it needs to be nerfed.

What really makes it great is its anti-wizard capability. There are lots of ways in which a 9th level wizard can shut down a mundane character - this is a welcome bit of turnabout. A wizard will be genuinely concerned if there is a character within 30 ft. who can use foil action. However there are several ways to counter it, including not getting hit by a ranged touch attack (either due to barriers, high touch AC, miss chances, mirror images, etc.) or by having multiple action types to employ (quickened spells, swift expeditious retreat, etc.).

So it is, imho, a great ability. But there are some problems with it.

One thing I find about foil action that makes it hard to adjudicate is its reliance on actions being declared. For example, a full attack action is never declared; someone makes an attack, and after it is resolved they decide if they want to make the bonus attacks granted by a high BAB. When does a mundane with foil action get to interrupt? Unless you are in a game where it is routine to say "as my standard action I... and with my move-equivalent I..." you'll probably run into trouble.

The other problem is with the fluff, which I think was best explained by Ice9 over on the Gaming Den boards:

There is kind of a fluff problem with Foil - the way it's described, it borders on "Captain Hobo" syndrome, which I'll explain.

Champions/HERO system is effect-based. That means that the strength of a power is determined by what you paid for it, and you can add whatever flavor you want to that. So Kid Saturn can have 12d6 gamma eye beams, and Battle Machine can have a 12d6 artillery barrage, and everyone's happy.

However, then there's Captain Hobo. He buys similar stats to everyone else, and then gives them extremely weak flavor. His attack is hitting people with a bent golf club, and his armor is cardboard boxes wrapped in duct tape. His reason for moving at SPD 5 is "too many energy drinks and vodka". Captain Hobo makes everyone's character lamer just by existing. Now it turns out your gamma eye beams do the same damage as a drunk guy with a golf club. Not very impressive.

Ok, it's rare anyone would actually play Captain Hobo, and if they did so in a non-joke campaign it would probably be vetoed. But the effect does happen to a lesser degree, just from differences in what people think is cool. Adam makes a non-super secret agent that's just that badass, and now Bob's beam of pure destruction has the same effect as a handgun. Not an insurmountable problem, but it does require getting on the same page from the start.

So, back to D&D. Foil Action, with the current fluff, has exactly this problem. You're Beowulf, the Berserker, the force of nature, the monster that destroys armies. And you wind up for the blow that will shatter a mountain ... and then some guy slaps you upside the head, or tosses sand in your face, and you're done, no check, just done. Badass factor ... gone.

If Foil Action was described as being more significant fluff-wise, as some kind of impressive action that looks capable of foiling just about anything, I think that'd be half the battle.

The other half is that level should matter. Two 9th-level characters should not be able to keep a 20th level one reliably stun-locked.

I think that the mechanics of foil action could be tinkered with to make it easier to use. For instance, it might only work against spells, spell-like abilities and supernatural abilities that take a standard action or longer to use. If you wanted to nerf it so that a DM could use a Big Bad in a lazy way, make it so that it is an automatic success only if the mundane has the Edge against the target (basically, has a higher BAB). Otherwise the target gets a will save vs DC 10+BAB or something.

As for Captain Hobo syndrome - well, I think that shortly after 5th level every character needs to define some sort of power source. If you want to pretend to be 100% real-world mundane for another level or two, fine. But for a ninth level character ability there has to be some non-Hobo explanation for how you do stuff like this; latent psychic abilities, divine lineage, favor of the God of Mundanes, whatever.