This is so awesome, and I only have the free trial version with like 5 choices for each category.
I wish there were a way to add joints to limbs--of course the full version may very well have that, or atleast limbs with more joints already. Because typically, 4-legged animals have 4 joints, since their metatarsals are really long. So with only 3 joints, you can only make rather odd-looking 4-legged animals.
I tried making some real life animals, but it was surprisingly hard (probably again because I only have the trial).
What's a lot of fun is seeing how they move based on your design. Like I made a horse-looking creature and thought he looked pretty cool, but then when moving, his joints were too stiff so he sort of ambled on peglegs. Then I made a creepy raindrop dinosaur with only two legs and he ran perfectly. I was really impressed with how intuitive it was.
I heard that you can't make fish-type or flying-type creatures, but some skins make a water "splash" when you paint them, and you get parts related to flying and swimming, so I wonder if the next expansion(s) will open up those possibilities. Of course I should just wait until September, when it's released :P