It was a play of two halves. A pacy, thoughtful, entertaining first half and then a second half seeking resolution, often where it wasn't needed. Some of the dialogue has developed a life outside the play. A line destined for books of quotations happens when Rudge, asked to define History, replies "it's just one fucking thing after another".
The louche Hector argues for Houseman's dictum "all knowledge is precious whether or not it serves the slightest human use". There are those who see this as being arguing for "knowledge for knowledge's sake", and what is wrong with that? I'm a strong believer in the existence of a moral imperative to know. After all, Ignorance was one of Beveridge's five great evils, the others being "Want, Idleness, Squalor, and Disease" standing in the way of post-Second World War reconstruction.
There is an alternative formulation, courtesy of the ICFI, that goes "All knowledge is precious and serves a human purpose whether or not its usefulness is immediately apparent”, but that would be different, misses the point of Hector's belief in knowledge for the sake of knowledge, and relates more to the temporal nature of knowledge, and that what is now pure research is tomorrow's applied science, is tomorrow's Simon Says.
Here's one excellent review. And here's another excellent review of the History Boys.
If you can, go see the play: if you can't, go see the movie.