I bought the accessory a while back so that I have at least two when I also get Wii Sports Resort, and I also rented Grand Slam Tennis. So far I'm not quite impressed. I was expecting every one of my movements to be mimicked on screen immediately (essentially, 1:1 movement), but apart from preparing for a shot, my character on screen only reacted after I did a full swing. If I swung on the wrong side of my body, my character would react by hitting in the right direction to get the ball.
Although I haven't played for long, it seems the Wii Motion Plus doesn't deliver that exact 1:1 movement, at least in this game. It could be the game that is messed up (and for its defence, the game is better with Wii Motion plus since it is better at detecting top spin and slicing), but until I play a better game, I do not have too much hope for the accesory.