Good, If You've Never Done It Before
I actually thought Demigod was pretty fun. I've played several Stardock titles, and they're not all great, but this game felt pretty solid. It is an obvious riff on the "Defense of the Ancients" types of strategy games, although it did suffer from a few major flaws.
For me, the number one reason I enjoyed this game was because it supported single player. DotA was always frustrating for me because I missed the initial rush of people getting on it, and so I've generally not been able to find games to join that were friendly to noobs. This was a game that let me have the gameplay experience while playing against computers. I had little interest in playing online with other people, and can't (and won't) review that experience.
The characters in Demigod are pretty distinct, although I felt like some characters were better or worse than others. The Assassin always felt incredibly weak because his mana and skills never did enough damage to actually win a fight on his own. I don't feel that the characters are well balanced against each other in general. On the website, there was an "Assassins vs. Generals" tournament, and the Assassins mopped up (by a lot).
With that said, the characters are each distinct, and most major RPG archetypes are represented. There are two official hero classes, Assassins and Generals. Assassins are powerful individuals, while Generals have the ability to buy additional summons. Generals can become powerful, but I think they get there more slowly because they have to spend a small fortune buying their extra summons, and without them they feel totally inferior. Unfortunately for the generals, the best skills in the game seemed to be the stat boosts to your hero, which are exclusive to the Assassins.
The best single player experience is the "tournament". In this, you are competing "individually" even though you're on two teams. There's a series of eight matches, played on the standard maps, and your individual performance is tracked through points. You earn accomplishments for things like capturing the most flags or killing the most enemy heroes. There's enough of these that if you were on the losing team but you carried your team the whole game, you can make up the difference in points from losing (although it's still pretty bad).
Overall, I thought Demigod was alright. I don't recommend you run out and buy it, but if you're looking for a single player DotA experience (like I was) then it can meet your needs. I ultimately find myself calling this game "Good, if you've never played one like it before."