Its definitely showing its age (it came out eight years ago), but its still fun and essential for any gamer.
For starters, if you've never played a PC RPG before, this game is a great way to start. It goes for $10 pretty much everywhere, and its extremely easy to learn. If you're a veteran RPG fan, you probably won't be that impressed by this game. The formula that was so new and brilliant in '96 has been improved upon by so many games, from its magnificent sequel to clones like Divine Divinity. In fact, compare to recent Diablo clones, the original Diablo is primitive: the graphics are dated, the gameplay is somewhat slow, and the tech system has been done better in practically every RPG since then. Also, the last few levels are unnecessarily hard.
However, the game is still a classic. It still has Blizzard quality. It still has a great dark fantasy feel. Clicking monsters to death was fun eight years ago, and its fun now. Just know that the game has since been topped.
GOOD: It's an undisputed classic, and its selling for $10; massive dungeons; entertaining gameplay; great atmosphere.
BAD: Primitive by today's standards; gets really really annoyingly frustratingly hard toward the end.
OVERALL: It won't blow you away like it blew people away in 1996, but you'll be entertained.