I've looked through the free preview.
Overall, it is very to understand.
How different (if at all) is the full version of the game from the free preview?
It's a huge amount of the game. It's only missing higher level spells and monsters. You could go up a few levels with this to find out if you like the game or not.
There are only 10 levels, and I think that this covers you up to about half way?
in addition, the full game includes a lot of extra tables and things, making it even easier to run on the fly.