Once we'd attached the add-on and locked it into place (there's a slider), the test could commence: would the game now be any more realistic than without it. It's worth noting that we were able to snap the MotionPlus dongle onto the Wiimote on-the-fly and the game instantly recognized and accepted input from it.
The verdict: it was ... alright. EA is heavily touting the added realism introduced by MotionPlus; we mostly found that we tended to miss the ball a lot. Putting top spin on when we did hit it was true-to-life, but, then again, so seemed the challenge level as a whole.
The demo was brief, and it's entirely likely that, given more time with it, the MotionPlus controls might have grown on us. We'll definitely have to see and play more of Grand Slam Tennis before it's served up in June to know if it's just us or if 1:1 accuracy really makes for a better time on the (virtual) court.