Not too bad
It was alright, with some thought put into making this different.
However, it just wasn't as fun to play as it could have been. There's not enough of a curve from the first to last level and it wasn't good the way that not only your but also the opponent's points advanced the level.
I'd have the speed of the ball increase as the game and levels go on, and if the difficulty only affects ball speed, just ask folk to select the ball speed, not difficulty.
Personally, I didn't really think the gravity fields were that fun an addition - whilst they did mean a bit more thought was needed to work out where the ball was going and make the game obviously different.
Was alright.