Extremely addictive and fun, though a little on the short side.
Punch-Out has a very simple concept-
Memorize your opponents moves and learn to counter them. You can bea your opponents in a couple different ways. For example, if you're a very offensive player you can learn the precise timing it takes to earn stars that allow you to do super punches.
If you're a defensive player you can simply dodge your opponent's attacks and throw some jabs in while he's recovering.
The best players will most likely use a balance of offensive and defensive tactics.
The story is also very simple. Each boxer has a short background story that introduces you to what kind of personality they have. There's plenty of funny stereotypes and even some unique personalities that will keep you entertained. There aren't too many unlikeable characters in Punch-Out.
The one complaint players may have with Wii Punch-Out is its short story mode. After about a dozen fights you'll have seen most of what the game has to offer. The game does hold quite a bit of replay value, simply because each opponent requires such different tactics. It's hard to get bored with Punch-Out.
The multiplayer mode is pretty bare-bones, but it provides enough fun to warrant a few rounds with a friend.
Overall:
Punch-Out is a great addition to the series and an amazing addition to anyone's Wii library. You'll likely find yourself coming back to it over and over again.