I'm going to jump in. I've read most of John Ringo's books, and while the quality is unequal, I'd say he's one of my favorite military / sf / space opera writer.
His series Troy Rising (Live Free or Die / Citadel / The Hot Gate ) is so far excellent, fast paced space opera, definitively a page turner. We will see if he runs out of ideas (I'm guessing more books are forthcoming), but so far each book is good if you like the genre.
The Posleen War / Cally's War is not his best work. While I read most of the books, you definitively get the feeling he ran out of ideas but had some bills to pay. Characters could be better developed, but there are some good battle scenes, and good ideas. Average space opera.
The Empire of Man Series (March Upcountry / March to the Sea / March to the Stars / We Few) is in my opinion his best work so far. The characters are well developed, the plot is excellent, and there are good battles scenes. I got attached to the characters, and when they die (the fourth book title is a hint) I got kind of sad, which is high praise. Military science-fiction (not space opera). I read the series three times so far and I'll probably read it again. The first two books can be found free at Baen, so it's a no-risk introduction to the author.
The Council Wars Series (There Will Be Dragons / Emerald Sea / Against the Tide / East of the Sun, West of the Moon) is another good series, more fantasy then SF, with a heavy military focus. While not its best work, it's nice. A bit on the preaching side, as noted in other comments.
And then we get to the Paladin of Shadows Series (Ghost / Kildar / Choosers of the Slain / Unto the Breach / A Deeper Blue / Tiger by the Tail (forthcoming)). This is the series that got everyone so mad at him because the main character is into BDSM. I'm not sure if I want to enter into the debate here, but let's just say that if this offends you you should not read this series. On my part, he got me to think about what, exactly, is a normal sexuality. That said, and allowing for some post September 11 Arab-bashing, the series is excellent. It's more of a contemporary thriller than SF or fantasy, with some heavy military undertone and fast paced action. Not his best work, but nice books to read while on vacation and you just want a page turner.
I have not read the Looking Glass Series, so I can't review this series.
I hope this helps.
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