Well, it's a classic but very much of its time. It is a Bildungsroman about Juan Rico, who volunteers for the spaceborne infantry and grows up fighting for humanity against the alien Bugs. The writing is pacy and entertaining; the twist in the tail is that Rico, having learnt about life and been tested repeatedly in combat, ends up as his own father's commanding officer.
There are two major problems with the book for today's reader. The first and simpler one is sex: there basically isn't any. Rico and his colleagues are all men except for the pilots who are all women. It is possible (and I think probably intended) to read all Rico's encounters with the opposite sex as purely platonic, though in a gentlemanly desirous sort of way. The only lasting relationships are between comrades. One can read all sorts of possibilities into that, but I will spare you.
The other, this being Heinlein, is more complex: it is the politics. In this future world, only veterans of the armed forces are full citizens with the right to vote, and this is an additional motivation for enlisting. Heinlein certainly mainstreams his political culture throughout the novel more convincingly than some of his imitators, complete with jibes at today's bleeding-heart liberals, but are we really meant to believe that this is his preferred alternative? One can read the evidence in various ways, but I think we can exclude the possibility that Heinlein actively thinks it is a bad idea, particularly given the uncritical admiration of the military way of life which permeates the narrative. Though perhaps it is at least in part a challenge to the reader to work it out for yourself.