Spore has an interesting concept, but apart from that, it does everything wrong
Editing the monsters itself is the good part about the gameplay. You can change the form of your creature, what legs, arms, tail etc. it has and also there is some strategy because each body part has some stats with it. By earning intelligence points and leveling up you can put on bigger body parts. You can try to make you creature look cool, but chances are you will have to think more of your stats and probably give it to sets of arms and legs and make it look rather freaky. If this had been a bigger part of the gameplay, like if you had a dozen different creatures, the game would have been better.
The rest of the game is nothing close to good. The story is lame and the missions are neither challenging nor fun. Combat does nothing else than replace button mashing with stylus slicing and have little or no strategy to it. Petting with new creatures isn't that great either. Most annoyingly is the camera angle, which is set too close, so I prefer to look at the map when I move. Also you need to walk in a big circle to turn the camera to look behind you.
The graphics could have been better. A 2D creature in 3D world worked well with Paper Mario, but it doesn't work in this game. The graphics aren't especially artistic either. And the music... My advice is to just turn it of.
Throughout the game, the motivation to continue playing is pretty little, and I'm pretty sure I will never pick the game up again. In a month or so, I'm not even sure I will remember it at all. So it isn't a game worth buying if you fancy good gameplay, but I guess for some, the creature editing is enough to make them enjoy the game.