I've had the Cortex core rules for a while now, and on the whole I like it. The basic mechanics are simple and very flexible--so it's easy to improvise in play, and very hackable outside play. Each attribute, skill, and trait is rated as a die size (d4, d8, etc.). When you attempt anything, you take the dice from each attribute, skill, and trait that applies, and roll them; add up the results, and compare the total to a target difficulty number, or to an opposing roll.
The "feel" of Cortex is a well-balanced mix of system light traditional play, and more of a story game approach. In particular, the character complications (disadvantages) and plot points form a powerful mechanic, similar to Aspects in FATE, which rewards both good roleplaying, and creative, player-driven plot input.
The core rules themselves are fairly bare (basically just traits, skills, and combat), although the book features a "Special Effects" section about adding Magic, Psychic powers, Cybernetics, and "Meta Powers" to the game. This is quite helpful for customizing it into the game you want.
If I have any complaints about this book, it's only these two:
1) the combat rules are actually a bit clunkier than I'd like for an otherwise very straight-forward game; for example, four types of physical damage, and a grappling rule I found difficult to decipher.
2) It's not very well supported. They avoided any sort of economy or extended equipment section in this book, and the Special Effects (magic, etc.) rules are really only short sketches of what's possible. To their credit, they are very upfront about this. Still, in a perfect world, I'd have liked to see some published supplements or a few genre books to support the core rules. You can import a few things from other Cortex products, like Serenity or BSG, but that only goes so far, depending on the game you're trying to run.
Case in point, in only a few days I'll be starting a new Cyberpunk genre game, and I've spent most of my free time for the past couple of months just putting together proper Martial Arts rules and an Equipment section--and in fact, I'm wasting precious time writing this review, because I'm not even close to done yet.
So, to sum up, the Cortex core rules aren't so much a great game, as they are a good kernel or framework for a great game. Lots of fun for aspiring system hackers, world-builders, and players who like some story game elements in their games, but not as well supported as some other generic games out there, if all you want to do is jump in and play.
[4 of 5 Stars!]