The game is definitely much more fun in multiplayer. Although the AI is decent-ish and can beat a human who has no idea how to play the game, the strategic depth of the game only becomes clear when you start playing other humans.
Games online can take anywhere from 15 minutes to an hour, sometimes longer if the losing side doesn't know when to give up. It's really not that bad.
The races are quite different and have extremely different play styles. (at least, good players use the different races very differently
)