Back when Virgin Media was NTL,
I was taking my barbarian through layers of Hell...
This classic game, along with Starcraft and the Warcraft series, was one of the considerable forces in cementing Blizzard as a gaming powerhouse. They've since become richer than God himself through World of Warcraft, and there's a lot of hype for their upcoming sequel, Diablo III.
Diablo II was a simple, charismatic isometric RPG with 5 levels (including the Lord of Destruction expansion) and each level had a series of quests, culminating in a boss fight at the end. This is all par for the course, but what made Diablo II so compelling was the looting - you run through the world looking for caverns and caves that you're yet to explore with one simple mission: find some new, fancy shit to hit monsters with. The random-item generation system is really well done, and when you do find something useful, it's a really satisfying feeling. This is, however, short-lived, as you then need to go and find other items to complete your set-up to a similar standard.
The game-play is as simple as it sounds, almost exclusively mouse-controlled, you navigate, move attack and arrange items within your inventory with a series of clicks and drags. This is all well and good, but the more modern numbered-skillbar-for-ease-of-use wouldn't have gone amiss in this title, it does suffer a bit from an unintuitive way to select which skills you would like to use, only being able to map them to either a left or right click. The other main criticism I have for Diablo II is that the online community is notorious for hacking this game, making everything all the less enjoyable unless you want to undertake the same endeavours yourself. All in all, though, this is still a great game and has done a lot for the genre, arguably being at least in part the spiritual inspiration for many games, including Borderlands, Gearbox's FPSRPG loot-a-thon.
Summary: Obviously a bit dated now, but still has a great many hours that can be played on it.
Recommendation: If you like the old isometric RPG style and haven't played it, give it a go. Especially if you're looking forward to Diablo III. If you're more into heavy action and skill based isometric titles, I'd stick with some RTS games like Starcraft, or the more similar Warcraft III.