I like Rifts Japan. It's everything that's good about Rifts: culture, guys with katanas, and giant robots.
The artwork is excellent, clean and uncluttered, lending itself to the text without being distracting because someone didn't know how to draw the human form (a big problem with some of the artists in past books). Rifts Japan is basically divided into two sections, the mystical stuff, and the technical stuff. I prefer the mystical stuff myself, but Rifts Japan is no slouch in the technical department either.
Unfortunately, the Millenium Tree information is reprinted, but that's a minor quibble. The Samurai and Ronin info are also drawn from other sources and given the Rifts treatment. Samurai are portrayed as masters of their mystical swords, and the ninja are basically more spiritual versions of their Ninjas and Superspies counterparts. It's the Bishamon, Sohei, and Yamabushi O.C.C.s where the book really shines. Each is unique, with rich backgrounds and powers to match. The Demon Queller, while appropriate, doesn't seem to be given his due (for a better treatment, buy Demon Hunter X). The Tengu make an entertaining R.C.C., but it seems curious that only Tengu were picked for inclusion in the book -- there's a huge amount of possible R.C.C.s that could be drawn from Japanese myth.
The next section, the tech section, contains the other side of Rifts Japan, the side that you see in all those William Gibson books. Here is the Cyber Samurai (people who act like samurais, not those cheesy Shadowrun imitators) as well as dragon cyrborgs and a host of nutty (and I mean NUTTY) ninja O.C.C.s. The cyborg enhancements, the power armor, all of them are uniquely Japanese in flavor, like the ArmaTech "Samurai Class" SAMAS Power Armor or the IPA-60 Tazu-Tengu Power Armor. Unfortunately, Glitterboys have managed to show up in this book too. Oh look, here's another goofy-looking camouflaged Glitterboy!
What Rifts Japan has that some of the other books lack is focus. The skills section, which often strays into irrelevant reprints of material, contains only a small selection of martial arts powers that apply to the O.C.C.s.
The book ends with a slim monster section, but all the creatures in there are impressive enough to give the players pause.
Overall, this is what a Rifts book should be. It maintains its focus on Japan without being filled with fluff reprinted from other games (okay, the ninja section is reprinted in every game since Teenage Mutant Ninja Turtles, with a picture of that one guy spinning the Manriki-Gusari) and manages to incorporate two very different elements of Japanese culture without doing either a disservice. Although I personally prefer the more mystical elements of Rifts Japan, it manages to be appealing to all kinds of Rifts players.