With each iteration of Scribblenauts, the gameplay gets more refined and more words and adjectives are added in. Apparently Scribblenauts Unlimited also has more of a story than the past games. At the core, Scribblenauts doesn't really change, but each new game is better.
For me it was a definite buy. Basically the game revolves around you exploring different areas of the world and doing tasks for people to get starite shards. An example of a task in the beginning is something along the lines of "Create a new fountain!" so I put a raccoon in the water and it immediately turned into a "Fountained Stoney Raccoon." Starites progress the story as well as unlock new levels. I think if you do all of the starite tasks you end up beating the story about half way through the actual levels. It was a good game, I got my moneys worth. (Played it on steam)
