I can't possibly do this book justice. I've barely skimmed the sections on martial-arts history, campaign ideas, or sample characters: all my time has been spent thinking about tactics with the new combat possibilities. New rules come mostly in the form of new combat techniques and options (Committed Attack, Defensive Grip, etc.) and are nicely modular so you can use only the ones that support the flavour your group is looking for (cinematic, chambara, unarmed, etc.) without an combinatoric increase in complexity. The new techniques are intended primarily as new combat options and only secondarily as something to spend points on--Jeet Kune Do specializes in feints and counterattacks, for instance, but it's made clear that that's as much a combat-roleplaying guideline as a rule for point-allocation (you may choose to just raise Karate instead of Karate/Counterattack technique, and that's fine). The section on Style Perks is really neat, and fixes some GURPS Basic annoyances (in Basic, buying Off-Hand-Weapon technique is crazy because it's always cheaper to buy ambidexterity; in GURPS: MA e.g. Off-Hand-Shortsword is a perk costing 1 point instead of a technique) as well as having some cool new perks (shtick: flicking blood off your sword as a free-action Intimidation attempt). In several cases, GURPS: MA supplies optional rules that let me throw out my house rules by providing the flavour I was looking for but couldn't create with GURPS: Campaigns rules (heroic D&D-style combat, for instance, with Chambara rules + Combinations + Counterattack + Targetted Attack + Multiple Blocks + Special Exercises for extra HP). At the same time, the principled approach toward new rules, such as favoring techniques over ad hoc special abilities for styles, avoids balance problems that plagued the old 3E approach toward martial arts while maintaining the "coolness" factor of super-skilled martial artists. (In 3E, for instance, Combinations were basically a too-cheap Extra Attack; in the new book Combinations are a variant of Rapid Strike which allows you to buy off the -6 penalty but are easier to defend against if used multiple times).
The book is packed with content and even better than the old 3E Martial Arts book by at least a factor of 2. When I put it together with rules from GURPS GULLIVER I have practically everything I want in terms of combat verisimilitude. The best GURPS 4E book I have seen yet.
-Max