"Hello Friend. Stay a while and listen".
Definetly worth playing. In the long shot it can become kinda boring grindfest (well, it's hack'n'slash after all) when you must kill 50+ pygmies just to pass that bottleneck. But the story is what makes that game great. You can just sink into it for a many hours. Variety of well designed monsters, various characters to play for different play styles, fantastic expansion, Lord of Destruction (which adds 2 more classes and final V Act). From the graphics point of view it is quite old and cinamatics aren't as stunning as current ones but the whole rest make it irrelevant. And don't forget about music... Simply stunning. I enjoy the most Harrogath Fortress one.
I'd recommend you to give D2 a try before D3 arrives. It will give you good play style and lore background.
If you can afford it the buy it. It is on of the best games if you like hack n slash. The way it is built around 3 difficulties shouldn't scare of people and if you look it at that way it can be compared to wows "Higher difficulty, higher gear" formula. It really comes down to the will of improving your character and be better rewarded for completing higher challenges. Single player offline with cheats is worthless and boring after 10 mins and the difference between easy mobs that dies with a whistle and those that barely dies with the best skills is the thing that makes normal and hell so far apart each other =)