As long as you are aware of and accept Diablo 3's online limitations, I wholeheartedly recommend it.
The rank-and-file monsters you face will be mostly familiar to Diablo veterans, ranging from zombies, spiders, and skeletons to minor demons and flying succubi. One of the most loathsome foes may be the corpulent Grotesque (and its variants) -- a fat, undead corpse that unleashes an explosion of attacking worms from its belly when defeated.
The strength of bosses, especially in later stages, could use some balancing, though. Just by doing some extra exploring to level up my character, spending some gold at the Blacksmith's to craft more potent magical armor and weapons, and augmenting my Follower with upgraded armaments and magical bonuses to complement my character's strengths, my Wizard was so obscenely powerful that I was able to slice through a Lesser Evil in all of five seconds. That feeling lasted up until my final encounter with Diablo, whose initial ass-kicking took me by surprise because his lieutenants had been such pushovers. Overall, Normal level was a little too easy -- and for some bizarre reason you're not allowed to dial it up.
Diablo 3 itself may be polished to a shine, but the same cannot be said of the infrastructure that supports it. As you may have heard (or suffered through yourself), the launch was spectacularly wretched, complete with errors and downed servers that kept players who had eagerly been awaiting the midnight green light twiddling their index fingers for most of the next day. Those used to the tumultuous launches of MMOs like World of Warcraft or the travails of Battle.net were annoyed, but ultimately shrugged their shoulders and said "Well, that's just Blizzard!"
In effect, Blizzard has forced the limitations of an MMO onto anyone playing Diablo 3 only in single-player. This means that when servers go down for maintenance (as they did numerous times during launch), you'll be kicked out of your game, even when playing alone. And while my experience was mostly smooth, reports of server-related lag making Diablo 3 nearly unplayable have been widespread. The question is, why is there server lag in a non-MMO in the first place?
Some may argue that Diablo 3 is intended to be an online multiplayer experience. In fact, the option "Allow Quick Join" is checked on by default, leaving the door open for your friends to join you at will. (Imagine my surprise when a friend abruptly joined during a big boss battle, replacing my Enchantress follower. It was NOT a happy surprise.) The problem is that, since Diablo 3 isn't marketed as an online-only game, there's a reasonable expectation that you'll be able to opt in or out of the online world.
As long as you are aware of and accept Diablo 3's online limitations, I wholeheartedly recommend it. A week after launch, the servers are up and running smoothly, though "scheduled maintenance" still brings them down from time to time, usually in the wee hours of the morning. But solo players who would rather live their gaming lives offline -- or have less than perfect faith in the stability of their internet connections -- should probably hold out for the offline-friendly Torchlight 2 instead.