Wow, for such a great series, I can't believe this thread gets so little traffic.
I should warn, there are SPOILERS for the entire series here.
I liked the first book, managed to get through the second, thought the third was great, and really enjoyed the fourth.
The RCs were all interesting enough, and Kal and Vau were straight up awesome. I was usually a little annoyed by Etain. I think her character would have been better served by the author if she had left the Order before or instead of Bardan. As it is, she kind of just went through the series with a target painted on her. It seemed like she had to stay with the Order just to die at the end.
I know a lot of people have a problem with the idea that there are so many Mandalorians running around in full armor when some sources said Boba was one of the last of his kind. I'm not really sure of the original source of this thinking, but I figure enough sources have implied there are more of them (going back as far as the Marvel run with Fenn Shysa and Tobbi Dala and including MandalMotors in Dark Horse, and new sources like Traviss's books) to allow that the original source of this idea might be discountable. Otherwise, I'm a Mandalorian dork, so I'm happy to see them all.
I was really happy to read in Order 66 that Jango knew full well the clone army was being made to kill the Jedi, and that that was the reason he allowed himself to be cloned, because he wanted revenge on the Jedi.
Otherwise, yes, Traviss is supposed to be writing Imperial Commando: 501st, which is set for August of 2009. I was a little worried when Order 66 left so much up in the air, so I looked into it and saw there were more books planned, at least two in the future.