Boo for not being Pathfinder Compatible!
The mechanics are not d20 but if this book is anything like the 3rd edition one then most of it is setting and background, which is easily put into any campaign you want to run, especially with the ninja and samurai coming in Ultimate Combat.
I will revisit the discussion after I have had a chance to read my copy this week(end).
I keep having the urge to write up some new cavalier (samurai) Orders: the Order of the Crab, the Order of the Crane, etc. Unfortunately, I'm afraid that when I get to the Order designed to act as bodyguards for their Clan's spellcasters, I will find myself the target of legal action from J. K. Rowling...
I've had a chance to go through my copy and I can tell you it is a good combination of fluff and crunch, I would say around 50/50. Obviously the rules are updated from the last one, so if that's what you're after it's worth it.
If you're looking for a book that would let you run the setting without the rules I think it has more than enough to make that work. It goes into various customs throughout the Empire and then breaks down how these customs vary by Clan. There is a lot of good setting information and even owning the 3rd edition book I felt there was a lot of new content in this one.