I have all three books, but I've only read through d20 Mecha. D20 Mecha is like the old Mekton Zeta construction system--it provides only the base mechanics for everything. If you have an idea in your head of what you want your mecha to be like, this is a great system. You can build pretty much anything you can imagine with it.
On the other hand, if you're looking for something a bit more specific at the cost of unlimited flexibility, Doomstriders and DragonMech might be the way to go.
I have mecha in my own campaign (which will be run next year). They are necromantic constructs, huge undead made from the corpses of titans. These undead are armored and have a chest cavity where the controller sits. They're more like enormous ride-along flesh golems than true mecha, but the latter will be created as a way to combat these constructs.
The next generation will see mecha replace the mounted knight as the elite noble warrior.