Great for the first run through (8 hours), then you've done everything in the game. Only thing new is your creature.
Civilisation phase was somewhat a let down. Trying to meld a lot of RTS characteristics into this phase made it seem very dilute. The trade, resource collecting, combat and city designing was all too simple. Intergalactic phase... well I just got really bored here. So many tutorials and to actually do so little.
Will Wright's vision of this game was amazing, its just that the implementation was terrible. The creature creation is definitely the highlight of the game, but how much long term entertainment can you gain from drag and dropping body parts? Each of the phases were too dilute in their gameplay. There was a lot of repetition of missions in each phase (kill these guys, ally these guys, kill the next ones).
This game would have been a LOT better if each phase had more depth. Maxis has gone for an epic feel with this game, which I kind of felt, but it was ruined by the simplicity and repetitiveness of the finer parts.