Seems to me its down to what your expectations are when you go into the game.
The Cell stage was short and sweet and a good introduction to the various aspects of evolution etc. The first time i got into the creature editor it was a joy to add bits on and similarly finding unlocks even in that 1st stage was great fun as it meant you had something to earn points for in order to add on.
Seeing my creature climb onto land for the 1st time was a proud moment and being able to advance your creature, only with more abilities and a proper world to explore was great fun! Its a theme that continued through the stages for me, always wanting to find more and watch my creature grow.
I can definately see how it wouldnt appeal to some though. The creative aspect of building your own avatar and trying to shape the world your in I great fun to some but a bore to others and similarly the game will in essence allow you to rush through it to the end game, something some again will want to do but which will make the game seem flat.
All I can say is for me its great fun, very original and a nice change of pace from most other games around at the moment. So my say is If you have a flare for diving into the creator and for taking your time exploring etc youll probably have a blast too!