I cannot begin to describe the value of this game.
Battle.net and friends to play with really adds to the replay value and turns the game from dungeon crawler into a treasure hunting adventure. You can spend near infinite hours in this game killing enemies over and over, trying to get that one awesome item. I found most of my fun in creating and playing various character builds, trying to see what crazy ways I could take advantage of my characters skillsets.
Diablo II also deserves a special mention for being one of the few games still playable on 28.8k dialup. (The forgotten connection speed.)
Definitely worth a buy, especially since you can get it with the expansion and original Diablo for $40.