As far as sports games go, I always prefer the action-oriented ones. Tennis and hockey translate particularly well to the video game world... all fast action and nonstop controller twitching. Since it seems unlikely that we'll ever see Mario Hockey, let's take a look at Mario Tennis.
Once again, Nintendo proves they know how to create the modern multiplayer game. With broad, colorful strokes, Mario Tennis turns this upper class sport into a knockdown, easy-to-acess brawl. The big-headed and insipid Mario characters drive the game, applying a fighting game style to tennis. Yoshi is fast, Peach is accurate, Bowser is strong, Mario is bland. Different courts offer different advantages to the various methods of play... better ball bounce and speed, for example. There is a certain science to selecting the right character for the right court, but it's not so devouring that you'll have to agonize over it.
The computer AI is acceptable, although some characters completely fall off the skill chart at the lower difficulty levels. In your first tournaments, hope you pull Luigi a lot. Mario's dopey brother can be faked out almost continually, as he likes to rush the net as soon as possible. (Leaving the backfield entirely open for you.) In the singles and doubles tourneys, the only real challenges lie in the last few matches. Typically, you'll be facing Wario or Bowser or Birdo.
(Aside: Who the hell is Birdo? And where did Baby Mario come from? Where are the goombas? And why is Donkey Kong Junior suddenly back? I thought he grew up to become the current Donkey Kong...) UPDATE: Since originally writing this, I have discovered that Birdo first appeared in Super Mario Brothers 2 and Baby Mario was in the Yoshi games. I usually don't add to reviews once they're "published," but I just couldn't let my ignorance stand.
Each tennis match is a smooth mish mash of double tapping the buttons. You use the A and B buttons for two generic types of shots. Double press gives your hit more power, and a neat comet trail to the ball. You can overhead smash by rushing to meet the ball and hitting both A and B simultaneously. After each shot, the game starts a dynamic cam replay, which you can quickly skip unless you're expecting something extraordinary. Thankfully, the replays do not slow down the game and cutting between replay and play is swift... unlike that dog Pokemon Stadium.
There is a selection of alternate gameplay styles, each saturated by Mario World dressing. The Piranha challenge forces you to endure a ball barrage by three ceaseless Piranha Plants. Ring Tournaments require you to aim your shots through huge floating rings. The Bowser court adds randomized Mario items to the game, so you can cannon a koopa shell across the court.
None of these are as fun and enduring as the "regular" style of play. I'm working on the Ring games now... I consider it the most interesting because you have to alter your tennis skull to succeed (you actually want to keep a nice volley going back and forth so you can brag the most rings.) The Piranha Plant seems needlessly difficult. The Bowser court is so confusing that it's tough to keep your eye on the ball. I guess that's part of the challenge, but I think the designers could have done the 'wacky Mario item' thang in a less distracting manner. (Especially in the sound department. When the game selects your item, you're treated to a nearless endless clicky-buzz noise that makes you think you're on The Price is Right.)
And here's a cool idea that dumbass kids everywhere ruined within fourteen minutes. mariotennis.com is running several internet competitions; to enter, you visit the website and write down an access code. When you enter that code into Mario Tennis, you get to play a ring-based match. When the game's over, you get a new code based on how many rings you tagged, which you then enter back into the website. Good ol' mariotennis.com then posts a leader board listing everybody's name and score. And good ol' jackasses the world over have already de-crypted codes signifying 9,999 rings. They're currently in first place. Tied.
I wonder if Nintendo ever gets any picketing by Italians about Mario. In Mario Tennis, his stereotypical clowning seems woefully irritating. In fact, it's to the point that other, presumably non-Italian, characters are crying out 'Mamma Mia!' when they lose a match. Mario's other questionable comments include "I'm-a gonna win!" "I am de best!" and "Please-a direct all complaints to my-a attorneys!" And, on the game's title screen, Mario's accent is so strong he seems to say "Mario Tennie! A ha ha!"
So I just have to say "This is one spicy Nintendo game!"