Definitely a fun and addicting game, but having more or less played through all of it, it is a bit disappointing in retrospect.
There's no real strategy for creature design, you just look at the stats of the appendages/etc, other than that everything is purely cosmetic. Nothing you do in each stage really helps you in the next stage. By the time you get to the last stage, all the 'design' becomes purely cosmetic and doesn't affect the gameplay, which takes much of the fun out of it, IMO. Maybe some people are more into the cosmetic side, but for me, the novelty of designing a weird-looking creature/car/boat/spacecraft/etc wore off by around the third stage, since it really didn't matter. As I've seen stated elsewhere, it's really 5 mini-games all rolled into one... and while the whole progression and continuity is really cool and immersive, when you stop and think about it, none of the individual stages is really as good as it could be, and for the most part they've all sort of been done better in other games - for example, the civilization stage is nothing but a really, really dumbed-down real-time strategy game.
Now I've gotten quite a way through the galactic stage and don't see all that much more point in continuing. I've unlocked most all the tools, so now I just have to go ahead and continue doing the same thing over and over to continue conquering the ridiculously huge universe, for no real gain other than to force myself to spend even MORE time running around stopping environmental disasters and pirate attacks on my existing planets which seem to be inhabited by defenseless sissies, and gaining more money which I can't really use for anything cool or fun, but rather just to continue my monotonous conquest.
It's definitely a really easy, intuitive game to learn to play, and very addicting and fun, but the dumbed-down simplicity really limits the depth in certain ways. There is SO much more potential for them to add depth and make the game so much better, IMO, and there are rumors that some future patches or expansion packs may take care of that... I will keep my fingers crossed.