Tremendously important book, from a historical point of view. But to be honest: Thucydides brings a boring story: he just gives a sequence of facts; no dramatic depth, no psychological dimension in the speeches, emphasis on the military events.
I also was a bit disappointed by his so-called objectivity: Book 1 is slightly anti-Athenian (imperialism), book 2 light pro-Athenian (Pericles). In comparison with Herodotus for me Thucydides is a little step back, because at least Herodotus gave different opinions, Thucydides leaves no room for uncertainty, he decides on the cause-effect relationship.