Spoilers behind the cut...
Basically, the book is Posleen Invasion fanfic expanded to novel length with the assistance of the author, and depicts the fighting in Central Europe between the Germans (mostly) the French (somewhat) and the Posleen invaders, focusing primarily on the doings of the 47th Panzer Korps. Unlike the rest of the rapidly expanded German Army, the 47th is organized using cadres of rejuvenated SS men, and this is where the problems with the book start.
It makes sense that even the SPD/Green government currently running Germany would do this, and the political consequences of doing it are well covered. It's obvious that either Kratman or Ringo (or both) is very familiar with Germany and its politics. However, one of the major characters (and a really obnoxious, evil bastard he is) does not make sense in this context. In contrast to the rest of the cadre we're shown, Sergeant-Major Krueger spent most of the war not in the combat units of the Waffen SS, but as a guard in the death camps. You can't ignore it, either. The character misses no opportunity to boast about his experiences, and other characters refer to him as disgusting and tolerated only for his tank driving skills (sic). This is a major lapse in the plot. It's made clear in other novels that priority for rejuvenation is given to combat veterans (the more decorated the better), and given the political damage done by the decision in the first place, what person in their right mind would aggravate that damage by rejuvenating someone with practically no value in combat? Aside from Krueger and his eventual nemesis Brasche, the rest of the SS men we see are just cardboard, going through the motions to fill their roles in the plot. Young, old, it makes little difference; the book isn't nearly long enough for you to develop much attachment to them.
Even Grueppenfuehrer Muehlenkampf doesn't really have much personality or description to him, and he's one of the main characters. One might wish that all the time spent on Isabelle de Gaullejac (sic) had been spent on one of the other young crewmen of Brasche's tank, or on Muehlenkampf.
The second major problem for me is how pointlessly gory the book is. You know from the get-go that any book dealing with the war against the Posleen is going to be horrific, especially in the scenes of battle - characters you've come to know and developed an attachment to are killed out of hand because the casualty levels (even in the elite ACS units) are just so damned high. However, there is such a thing as overdoing the horror. There are a number of scenes in Watch On The Rhine that are just gratuitous horror, done for pure shock effect and without the excuse of advancing the plot. Yes, we know the Posleen eat humans and their own dead; do we need to dwell on this at several points in the novel to the detriment of the main plot? Evidently Kratman and Ringo thought so, because there are a number of death scenes in the book that are drawn out in disgusting detail, Gudrun's death in particular.
Finally, the authors forget David Drake's advice on explaining one's work (Don't!) and insert an afterword explaining that the book is intended to shock and remind us of the unpleasant necessities involved in fighting the Islamofascisti. Uh, thanks, guys, but you do realize you're preaching to the choir, right? Most of the people who read combat SF are not likely to be soft libertarians or antiwar types, who aren't likely to buy your argument to start with, and secondly the analogy between the Posleen War and the GWOT we're currently engaged in doesn't work very well. Worst of all, polemics tend to suck as fiction because you're so busy trying to drive your point home that you tend to skimp on little things like plot, characterization, and all the other things that make books worth reading. This is why the SF works of Jack London and H.G. Wells have not stood the test of time very well (I find them both damn near unreadable at this point) to say nothing of various works of propaganda (left and right) disguised as SF that came out during the Vietnam War.
In summary, this is a disappointing book. If you enjoy seeing cardboard characters get what they deserve - not typical in Ringo's other novels in this series, BTW - it's an okay read, but there are a lot of parts to Watch On The Rhine that are just flat-out bad. I should have waited for the paperback, and bought it used at that.