First off I have to admit some bias in looking at this product. It's for a cause that's close to my heart, and I would probably encourage every RPGNow customer to tack this on to their next order, even if the PDF was just twenty pages of unintelligible gibberish.
Fortunately, that's not what you get. What you get is a pretty nice generic little RPG system. As the name suggests, F20's roots lie partly in D20, but it's got a bit of a Fuzion/Action flavour in places (and to be fair, the author credits the games he's used as inspiration)
F20 has four stat-groups, Body, Mind, Grace and Spirit, each divided into three substats. Now I'm not a big fan of tightly focussed game stats (although I understand the appeal) and F20 has an option to just use use the four groups as stats instead. There's the usual Skills, Advantages and Disadvantages sections, with a good selection in each. The base game mechanic is Stat + Skill Vs Difficulty as in core D20, and F20 even ports across the Taking 10 and 20 rules. Combat is similarly stripped down D20, except damage is based on multiples of D6, and spread between Stun, Hits and Mortal damage. The PDF rounds off with a Powers system, which allows you to tailor a set of stock powers with modifiers.
There is one, staggeringly glaring omission however. There is no equipment or weapon table, so the author gives us no clues as to how much damage various sorts of weapons would do. You could argue that since this is an entirely generic ruleset, it's up to the GM to tailor weapons to suit the game style, but even so we really need some sort of guideline to work off. (Judging by the Electricity & Fire damage table, weapon damage ranging between 2d6 and 8d6 would be about right, and other stats like range increments and crit ranges could be lifted from D20 games)
The layout of F20 is two columns of plain black text on a white background, with no artwork; very minimalistic but very clear and easy to read. The writing is good; solid and concise for the most part with a couple of lighter passages. Overall, F20's a much better job than any homebrew I've seen, and better than quite a few "professional" PDFs out there.
F20's biggest weakness is that there's nothing about it that makes it stand out and scream "play me". I think it would make a good, simpler replacement for D20 in a lot of cases, but there are other systems out there that do the same thing. I can't see myself running F20 in the near future, but it's an interesting fusion of game systems and I wouldn't begrudge the cost even if it hadn't been for a good cause.
The second biggest weakness is that it really isn't finished. It's listed as version 0.9, and true enough it would require that 0.1 of GM input (statting up weapons and armour etc) in order to produce a playable game. If like me you always do that 0.1 work anyway to adapt commercial games to your campaign, this shouldn't be a problem.
In summary, a professionally presented, crunchy bit of homebrew game design with its roots in a couple of mainstream games. Worth looking at, both for interests sake and also as a way to support a good cause.