If you're looking for an exciting roleplaying game, and you have the basic Mekton Zeta book this book is a godsend. It exponentially increases your ability to create mecha, as well as giving you several fun and very anime things you can add if you wish (such as psionic powers).
The only real problem I have with this book is that it can be very math intensive. It takes a lot of scratch paper, re-reading certain parts a couple times and more than a few restarts to finally get the creation process down pat (there are guides and examples but it's still difficult). However, once you've got it down, it is worth it.
This book also allows for less serious mekton campaigns including a section called "Cheesey Mekton Tricks" which can duplicate everything from indestructable giant headed small bodied goofy mechs to power rangers, if that kind of stuff is your thing.
The psionics previously mentioned are cool, they fit pretty well into the scheme of things (I don't personally care for having to add a stat to all psions that non-psions don't have but oh well, I never said it was perfect) and allows for some nifty (though not truely exceptional) alien races.
Oh, and if you're the referee you have to check out the section on "Creating totally unbalanced, godlike aliens." It's priceless.