Many (author included) have noted the similarities between Old Man's War and Heinlein's Starship Troopers, and the similarities certainly are quite strong. We begin with Perry's enlistment, then his training, and finally his deployment. The major difference, in this case, is that all the soldiers are 75 years old (hence, Old Man's War). 
 I'll start with the bad: Characters aren't very fleshed out. They mostly have the same, snarky voice, and anyone who deviates from this is both a) an unlikable bigot, and b) going to die within a couple of pages. Seriously, Scalzi introduces a mean character, or a character who challenges the group in some way, and then doesn't seem to know what to do with him and just kills him off. The only characters allowed to live are either Perry's friends, or his superiors who come off as hard-asses but give him the respect he earns. *yawn* 
 The characters don't feel like 75 year olds. Except for the occasional revelation from someone with relevant past experience, they don't seem to act or think like 75 year olds. This comes through as Scalzi has to keep reminding us, by explicitly telling us, that they are elderly, because otherwise it would be far too easy to forget. Rather than 75 year olds placed in young people's bodies, the characters feel more like 20-30 year olds who have unidimentional and largely dispassionate memories of someone else's experiences. 
 This is made all the worse because there's really no reason for them to be elderly. There's a brief mention about wanting people with a lot of life experience so that they are in the right mindset to fight and die for humanity, but 20 year olds fight and die all the time, so the logic seems rather thing. The other reason mentioned is that 75 year olds have lived out their usefulness on earth. Okay, let's say I buy that, why does the CDF care? Why not an open enlistment? Why only 75 year olds, and not 65 or 80 year olds? 
 Overall, the age thing feels like a gimmick slapped on over the top of the story, rather than the integral part of it that it should have been. The only place where it really comes into play is with Perry's wife, but Scalzi could have killed a 30 year old's wife in a sudden accident and not needed to change the story - or his main character - by any significant amount. 
 I was also a little bothered by how easily the soldiers adjusted to being green. It's described as not chartreuse, but rather more of a greenish tinge, which sounds like it should make people look like corpses. Yet except for briefly mentioning their green-ness from time to time, no one ever seems bothered. 
 And that's really where my complaints end. The book is well written and a fun read. Even with the characterizations lacking, the action was entertaining enough to keep me turning the page. I also found the setting very interesting - the different intelligent species Perry encounters (and fights), the technology, etc. The BrainPal, in particular, is a wonderful idea, and reasonably well executed. 
 I've seen a few complaints that Perry seems to be a little too perfect, in that he's always figuring everything out way ahead of everyone and such. But what I liked about this is that he'll have some epiphany, figure out some new strategy or piece of information that helps him in the immediate moment, then finds out that his superiors have known about it and been using it for a while. And that meshes with my experiences - a reasonably creative thinker will come up with many innovative ideas, but truly unique ones are rarer. The only time Perry is the first to come up with an idea, it's in a brand new situation where someone was going to be first. That's pretty inoffensive, as far as Mary Sueisms go, I think. 
 It's a fun read, a good way to waste away an afternoon. What it lacks in depth, it makes up for in creative ideas and entertaining action.