I think D3 is a very good strategy game. There is so much content that it seems like the developers threw in any magic thing they could think of, but it is actually all quite useful and balanced. There are a good variety of strategies available, based on what nation you choose, how you design your god, hidden magic sites you find during the game, your research path, and counters to what your opponent is doing. The 300 page manual is daunting, but the game is not too hard to play because most things work in intuitive ways. Although if you want to play well, you'll have to look at the manual for details and math formulas on how everything works. I think the demo gives a good impression of the game because it includes everything except the high level spells and most of the nations.
For cons, the interface isn't very convenient, though it works ok once you learn the keyboard shortcuts. You kind of have to choose your strategy before the game starts when designing your pretender, and changing your strategy after the game starts puts you at a disadvantage. I think you'll get the most out of the game by playing multiplayer. The game has an interesting transition where gold and soldiers are important in the early game, but in the late game, mages, magic gems, summoned creatures, and forged items are more powerful, but the AI is only competent at the first half of the game. So multiplayer is probably the better game, but I haven't played it because games can take months. I loved D3 while I was playing it, but now that I can beat the AI, I'm pretty much done with it.