Well, I just finished Order 66, and let me say that this series has lost the potential the first book gave it. Don't get me wrong, it's still good, but not as good as it could've been. Triple Zero was where it wrong in my opinion. When Traviss started focusing too much on Kal Skirata, and getting clones to desert, it no longer had the feel of being a Repuclic Commando novel. More like Republic Deserter.
Anyways, my recommendations are: read the first book definitely. It truly dealt with commandos doing their job, and how they did it. Then, just prepare yourself for the other books to throw that out the window, and read on with the new type of storyline.
Anyways, Order 66 was really good. It was better than Triple Zero and True Colors to say the least. Though, it still focused too much on Skirata and his plans, and not on RCs. But, if you throw your expectations out the window, and just try to enjoy the book, you will enjoy it. It's detailed and suspenseful.
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Also, the end with Etain's death, and everyone mourning her, and Darman and Niner still on Coruscant, and Mandalore (planet) going downhill because of the Empire was very well written. It was very sad, and almost made me want to cry.
So I'd definitely give this book a thumbs up, and recommend to all you SW readers.