As others have said (and with good reason) Europa Universalis is a classic, the game that more or less started a genre.
Graphics-wise: Nothing special to look at, looks somewhat like a board game on a computer.
Replayibility: This is one of the strong suites of games developed by Paradox. While the nation choice is very limited compared to the later games in the series, it nevertheless has well over a dozen playable nations, all of which having different advantages and disadvantages from each other, enough to keep someone who enjoys the genre entertained for many, many hours. I would say that it captures the "Just one more turn!" mentality of the Civilization games, in other words.
Music: The music is decent enough, but since you doubtless are going to be playing it for many hours the music will likely get old fast.
Gameplay: You get many options as to when you desire to start, ranging from the mid-1400s to the end of the American Revolution. Within these starts there are many nations, each with numerous ways to "win". Do you want to become rich purely by trading? Do you want to settle a massive colonial empire? Do you want to bring civilization to the "heathen" population of the eastern hemisphere? Or do you want to expand your empire the good ol' fashioned way - blood and iron? All these options are available, and a few more besides.
Multiplayer: The multiplayer plays much the same as the singleplayer, with the addition of the fact that you can no longer count on having played 1000+ hours and thus knowing exactly how the AI will react to every action you take, et cetera. There is also a scenario that takes place on the world map, with 8 or so factions, each controlling one province and the rest controlled by nobody. This is as similar to typical real time strategy games as games of EU get, as you could colonize distant lands early game, or you could focus on consolidating your homelands.
Historical Accuracy - While the game itself starts the nations for the most part as they were during the era, what happens after game start is anyone's guess. The Ottoman Empire could be crushed by an alliance of Russia and Britain, or on the other hand the Turkish thread could push north for hundreds of years, until it controls land up to Denmark. In addition, historical events occur occasionally - the protestant revolution and the reformation of the Church, to name just two.
Overall, Europa Universalis is a great game - and at a mere 1000 blue coins (or 500 at the current sale price) if you felt even the slightest interest in it you owe it to yourself to pick it up. While it may not be for everyone, those who enjoy it will get many hours out of it.