R-ball is GREAT! Love it. Perfect for me in the winter b/c I don't have enough money for indoor tennis clubs. It does mess with your strokes a bit, though.
Some benefits for tennis game:
(1) You learn to hit flatter a bit better.
(2) It really improves your reflexes and reaction time while holding a raquet. (Good for volleys, taking the ball earlier.)
(3) (I think) it keeps your sprinting speed at a high level.
It was weird. Playing r-ball made me start playing with a one-hander BH in tennis. Just started to feel more natural, though I do still use 2 hands for returns and high balls.
And yeah: watch your follow through on the r-ball court. It's hard not to do full swings coming from a tennis background.