This game is not bad if you like clicking on monsters again and again. It is really fun if you got nothing else to play or want to kill some time.
Diablo got an expansion called Hellfire. It's made by Sierra. However this expansion got nothing to do with the original storyline and you can't go into battlenet with this expansion.
Diablo 2 got an expansion called Lord of Destruction, this is made by Blizzard themselves. This is a continuation of what Diablo 2 have left. Which is a must have if you want to know the storyline of the game.
And *cough* I got 2 copies of the battlechest (Diablo,Diablo 2 + LoD) that I'm selling for cheap. Interested?