I downloaded this expecting to be greatly dissatisfied - core rules systems don't usually impress me, but at fifty cents I figured there was no harm.
Wow! Action! is easy to understand, yet reasonably comprehensive. Also, unlike some other companies who are releasing their core rules here at reduced prices, GRG claims to be offering license to use their system for free, a boon to beginning authors.
Action uses a straight forward 3d6+Attribute+Skill roll vs a set DC, and shows some signs of GRG's former association with Fuzion (which in turn had a brief alliance with Hero Games.) You get a set number of points for attributes, and a separate pool for skills, advantages, disadvantages, and in high-powered campaigns: powers.
It's combat system is very well done, though in settings where armor may not be appropriate (modern, or Trek-kish, or similar) it can be brutal.
By itself, Action! may not be enough to run on for GMs used to a small library of information at their disposal. It's a core system, much like GURPS or Champions/Hero, and therefore the GM has to add any detail. (Though it does come with complete and reasonable skill lists for a fantasy, modern and sci-fi campaign.)
I think it's well worth the money!