Think of it this way,
you aren't buying it because it will be free when it is officially released. you are donating $15 to support the development of the game also while gaining access to development testing, which while still awesome, is a watered down version of what the full game will be.
There are a total of 3 parks; the main park on Sialia, the Pipe and park next to it on Sialia, and the new Fochi park map which is a remake of the Sochi course. There are also some fun hidden areas around the mountain like a castle and the T Hall invitational setup which i found yesterday. The mountain itself Sialia, is huge, and I have barely explored it myself so I can't comment much on this.
All in all I am really happy with how the game runs for me, i have an AMD radeon hd 7700 graphics card which isn't high end and i can run the game on high quality fine. I definitely think it's worth the $15 donation to help keep them funded and just to enjoy the game early.
