hedwards: Diablo has basically no replay value and is pretty mindless. That being said, I'm sure it was cheap and probably worth the bargain, but it pales in comparison to Diablo II or anything more recent.
WarZombie: For the sake of being fair to Diablo, I wouldn't compare it to anything made today because, I mean let's be honest, games have changed. As for Diablo II, I really hope it isn't as bad as you say it is. I'd be willing to give a try, but is it really that much worse?
I was going to just completely slam the game, but if you can pick it up for a couple bucks it's probably worth that. But the only interesting thing about it is the gaming history it represents. The aesthetic and mood are great, but you can't build a game completely on those two things. Diablo II was infinitely better in pretty much every other way and does have some replay value to it.
You'd have to play it to see what I mean, but there's very, very little reason to play the game through more than once. There are a small number of quests that show up randomly and 3 classes, but considering the near complete lack of diversity or variety I never bothered.
It's mostly just a tedious click fest that's more of an OCD ritual than actual game. At the time it wasn't as obvious because the graphics were good for the time and there weren't better options available.