re: full game: I think that I've just about got my money's worth playing through the campaigns for the different factions - but probably only just. But at least I was interested in completing single player - most games I just get turned off about three quarters through.
Downloaded CnC3 demo and watched someone else playing it for a few minutes yesterday and that looks much more fun - person playing said that they were enjoying it more than SC.
For anyone undecided about buying either SC or CnC3, it might be a good idea to try both demos first.