The REAL Diablo III.
The Graphics are nice, a little on the cartoony-side, but as a lot of other games have shown, the less realistic they are, the better they age with the march of technology. They get the job done and do better then it's illegitimate cousin who's stolen the family name.
Story is alright. Both games have a lot of parallels to Diablo and Diablo 2, so long time Diablo players won't see much in the way of innovation, but otherwise is still pretty solid. The setting could be described as Steam-punk-magitek version of Diablo, which is actually quite interesting in it's own right.
The game is EXTREMELY friendly to modding, in fact the developers actively encourage it, adding infinite replayablity for the future as new classes or balance changes, or even whole new quests and levels are created. It also doesn't require online save for multiplayer or to update and even allows LAN play for those that just want to play with friends without having to deal with internet issues...or DOWNTIME...
And the key point of any Diablo-style game...the Loot-whoring is bang on. You find lots of interesting pieces, and can customize to a large degree items via enchantments from various vendors, adding sockets, transmuting, several dozen sets and hundreds of unique items, and unlike a lot of other games, the drops are done for each person, so tackling bosses with more people is encouraged, since you're more likely to get 1 or multiple copies of rare items (though it doesn't tell you what other players get, so be sure to get some agreed upon loot rules (if any uniques drop, hollar!, etc)).
The pet mechanic is interesting by itself, handy as a guard, extra dps, storage, or just ditching vendor trash without having to pop back to town. A nice touch in it's base form, Mods have truly opened it up, allowing for dozens of different pets, or even Cohort-style other adventurers as pets.
It has a few minor annoyances, but none that really detract from the experience, and is definitely worth the money you spend on it, unlike it's chief competitor. If you're not sure, then wait till it goes on sale on Steam and grab it then, as even if you still don't really like it enough for multiple playthroughs, you've MORE then gotten your money's worth.