Incredible. You will be playing this for hours on end...
The only cons are the sometimes awkward movements of the creatures, and the limits on total size and complexity. Also the forced Bilateral symmetry gets a little annoying after a while.
I've personally made 25 creatures so far, just going over and editing until they look good, but if you want you could churn out hundreds in a week or so. So it's up to you on the time required to make a creation.
All around great game, worth any penny. Makes you wonder how awesome Spore will be...