Wow, just wow.
I was totally addicted to both I and II. I love DND/FRP style games, and Diablo had it all. Their game engine was so cool because you could dodge enemy spells and projectiles, unlike traditional FRPs.
As for II, people got into it so much they detailed all the algorithms for all the character types. It got to the point where there were formulas for how to create a fast cast sorceress, for example, by wearing all the right gear and putting your skill points in the right places in the tree. Another example is the tank sorceress, who basically was able to walk around slow as molasses and enemies could hardly damage her; the strategy was to attract as many enemies as possible and then rain down meteor strikes on the whole pack.