If ever there was a game just silly enough to sell systems this is the one. I can imagine that this is more popular next to the lava lamp at parties. As a game Mario Tennis is just about the silliest game of tennis since ... well, it's just about the silliest game of tennis, period.
What is so great, as well as it's greatest weakness, is the 3D elements of the game. As a show piece for what the system can do, it works wonders. However, in the game play elements of the game, the angle in which the 3D movement is distracting, and accounts for many missed rallies. This is not a picky item, either, it's noticable, and while you can over come it, it really detracts from the ultimate game.
The addition of the popular characters is nice, however, it seems a little trite now. Of course, I may be looking at this all wrong, as this game preceeded many examples of similar "Mario _____" titles. But none of the characters are extremely interesting, and I would have liked to have been Link or Samus. The animation is crisp, however, again not very interesting.
It's not that the game is bad, it's a fun little game, but it's difficult to play for an extended amount of time, and pulls off as nothing more than a novelty item. It's a shame, this game had real potential.