I personally think dominions has a pretty tough AI as far as games go. Until you learn all the AI-stomping tricks, you can expect to be beaten... repeatedly. I can think of plenty of games, RTS or TBS with much easier AIs to defeat. Of course when you know how to play, the AI is completely useless.
I still play SP, and have played SP almost every day since before I signed up for the forums
. I've probably logged similar hours playing dominions as many people do playing MMORPGs. And I've played in a number of MP games as well. So I think a person can easily get their money's worth just playing dominions SP, though from what I've seen most people who try mp have a lot of trouble going back to SP.
I'm someone who doesn't usually like to play strategy games multiplayer, but I still find myself enjoying dominions in multiplayer. I would sggest that even the most hardcore sp person try it at least once. It's a completely different game.