I read this book first many years ago and enjoyed it, in amongst many other books I was reading at the time but did not, looking back really understand it fully. Later I saw the film and was a bit disappointed, although it was worth watching.
This one, like many Heinlein books is a simple story, beautifully told; essentially of a relatively ordinary person, decent, mostly honest and good hearted and shows how he grows as a person on overcoming the challenges to his existence. Its worth reading just to enjoy a good story and the adventures that 'Johnny' goes through as he joins up hoping to do a couple of years to earn his citizenship and ends up committing his future to the cause.
One of the many political points raised is that in most societies today we have people voting bread and circusses. Basically, anyone has a vote and most have only self-interest in mind. Thus, in the UK political parties tend to pander to people to get elected and when in power have to do the same, perhaps rather than taking the decisions that should really be taken. As soon as the popularity ratings drop, a panic is triggered.
In other countries, military service is enforced on everyone, eg Israel, some others too. At least in Heinlein's world people have a choice. Its somewhat ideal and primary coloured; but if we were able to build a system based on natural law what would it be like? It would be simple, probably unfair but workable. Fairness is often screamed for, but who does one really want to be fair to, the guy who just claims benefit and contributes nothing; or someone who is willing to help improve things?
Now, I wouldn't run off and join the army after reading this again. But I do understand from this what the training is supposed to achieve and that a growing appreciation of real hard work, discipline and decency, working with other people and depending on them is actually what makes a person fit to be a citizen -- not just where one happened to be born.
The subjects this book takes up are big subjects and important concepts that a person should be exposed to and it provokes thought without being too complex for a young person to understand. Even if one doesn't agree, or perhaps especially if one doesn't the exposure to a different point of view is very valuable.
There are things worth fighting for and books well worth reading. This is one of them.