Summary: Okay graphics, but doens't let you design your own player. The remote doesn't seem to sense exact movements. But you get some exercise or it's a good warm up to another more serious workout. Not great, but okay.
Summary: I waited a few days and read a few brief reviews. Then rented both games and purchased the Wii Motion Plus to see which I liked better. Virtua Tennis (VT) is more realistic, has better game play and has a much better career mode. Like any EA game the online set up seems better and easier to find opponents. Not saying I did not find opponents on VT but there were not as many. Grand Slam was almost too easy to play and childish.
Conclusion: I like this game, it looks great and for the most part plays really well, however, playing backhand shots without the nunchucka is a nightmare. It perceives your arm movement into the backhand position as a shot which makes you miss the ball by a mile. Really REALLY frustrating.