Very relaxing - even failing the challenges felt fine. The combination of the music, art style and simplicity of the game is most likely the cause of this feeling.
It feels very complete and polished. The curling stones physics felt fine. I was initially confused when a secondary collision didn't cause a colour change, but I figured that was intentional in order to prevent a lot of potential steamrolling scenarios when a lot of stones were clustered together in the final turn.
The only thing that could be added to it would be a method to select a specific stage in the challenge menu - There was a conspicuous gap where it felt as though the stages could be selected. Then the player can complete the challenges in their leisure or even replay old challenges to improve their score.
I like the multiplayer option, though I feel as though the challenges should be given top billing over the versus mode.
A pleasant and relaxing game.