Punch people! What's not fun about that?
When this was released, I remember seeing ads for it and being reminded of just how long it had been since I had owned or even played a boxing game. They're almost always a fun little diversion. So I went down to the local game shop and picked it up. I pretty much got what I expected.
Its pretty much like any boxing game before it in that you spend time climbing the ranks to the title fight and win the title. Some games end here (Punch Out!), but this one allows you to continue your career and fight an endless onslaught of challengers until you inevitably fall from grace.
Why will you eventually lose? Because the game includes an "age factor". After your boxer hits his late 30's or early 40's, he will begin to degrade, regardless of how much training you put him through. You'll begin to lose punching power and speed, and take more damage from punches.
Of course, it isn't completely impossible to continue to win. I did it for maybe 10 years beyond the old age point, before finally letting the other guy win. But the fights literally become tests of endurance for you, the player. If you're a true iron man, you may be able to make to the maximum age, at which point the game forces you to retire.
The multiplayer is quite fun as well.
The biggest downside is that you'll be hard-pressed to enjoy this game enough to continue playing after 2 or 3 careers. There simply isn't enough content to provide that kind of replayability.
At this point, you'll likely only find used copies, and it should be dirt cheap, but if you're looking for several hours of simple fun, this game should be right down your alley.