Well, I've become a big fan of the system after buying a week ago during a con. I've built quite a few mechas, both for fun, and to compare it's results to those I find in already published games.
And, from what I've discovered, the Purchase DC rules need a bit of a tweek (though I'm not sure how exactly), the US$ and Purchase DC paragraphs need to be cross-checked (one says extra for space/underwater, the other space/flying). Also, either the credits rules are way off, or the LAAT/i Attack Gunship in the SW:RCR is way too cheap. (By an order of magnitude).
But, aside from that, I plan on using it to contruct some armored cars in my d20M game, and heopfully an armored vehicle or two for my d20SW game (though I'm gonna have to compare the SW and D20Mecha prices to get the price down).
Also, if you have the DP9 book, there is no need to get the GoO book, unless either you want a lighter, thinner book, or need a loaner copy for your players that doesn't have any mecha examples you're using in it. I plan on buying the DP9 book eventually, as well as the GoO d20 Mecha Military Vehicle book coming out this year (when I don't know).