Open Core takes all the best and juciest bits of OGC and brings them together, actually successfully melds them seamlessly, and writes an entirely new rules mechanic around them that makes the whole thing work extremely well. It isn't D20. It is, in fact, the newest Linux system upgrade for D20 - all the best things of what's come out so far wrapped into one package that looks somewhat different from the original but is far superior and retains all the best aspects.
Before I go into all the things I liked about it, I will mention all the things I didn't like about it. First of all, I have sundry personal style issues with the game (for instance, I would work the Ability of Special Attacks differently). Most of the issues I have of personal style I will omit, save to say that I am less "rules light" than most people.
There are also numerous sometimes-confusing editing mistakes (such as the CP to Skill points conversion: first of all, it's 1:5, not 5:1, and secondly is it 1:5 or 1:10? ...I'm pretty sure it's 1:5). This also includes a number of copy/paste errors (references to Saves, or to traits that are not part of the main Open Core system).
Now, on to the likeable parts, which is most of it. The system is modular; adding and taking away parts are easy. There are even descriptions of ways to add new traits, a genericized Skill list, and descirptions of how "everyman skills" should work (my term, not theirs). Feats and Special Attacks/Qualities are all brought together into one heading, Abilities (pulled from BESM D20 Attributes). There is also a long list of optional elements that can be added, just to show a few examples.
If you're particularly prolific in purchasing OGC material, you will recognize half of what you see. While sometimes a bit startling, I do not find this to be a fault. This system is not designed to be innovative - it's designed to be the best of what has worked in the past. In this, it is a good purchase, well worth the money, and better than many other "alternate D20" or "alternative to D20" games on the market now.
[4 of 5 Stars!]