First of all, I've played both "Spore" for PC, and "Spore Creatures" for DS. Spore Creatures wins hands down. Without even taking into account the whole DRM thing, this is much more enjoyable.
It plays similar to any RPG - complete goals, retain health, improve yourself through collecting and improving - but has quite a nice story to go with it, your creature's friend has been kidnapped by an alien and you need to find him. The animations are pretty cute, there are buyable cheats through game points, and it's generally good, clean fun. Anyone from the age of 8 should get on with this just fine, although the fighting isn't explained very well to begin with.
The playtime is also fairly long as well, I've spent more time on this than Spore for PC and I've pretty much finished the latter.#
Buy this if you want a nice, straightforward game for your DS and don't already have Pokemon.