This classic did not disappoint me. It is well-written: clear and easy to read. It's very long (the war lasted 27 years!) and full of details about who led what military forces, who said what, etc. that for me, and I think for most readers today, are simply irrelevant. But those parts can be very easily skipped; the excellent paragraphing helps a lot in this respect.
What is fascinating about this history is what it shows about the Greek civilization at that time. In particular, as Thucidides states explicitly, it shows how avarice, distrust, and the "us-them" conflict omnipresent in every human society combine to produce endless war -- laying waste the land (of an agricultural civilization), killing each other, murdering captured males and enslaving their women and children, and breaking or planning to break alliances as soon (or before) the oaths to uphold them are sworn. It gives us yet another demonstration that "plus ça change, plus c'est la meme chose" -- We (the human race) know lots about our history and yet we continue to repeat it. A broader lesson is the divergence between the ostensible ethics of a society and its actual practices -- again something that continues, perhaps exacerbated, to this day.