Great for casual gaming and a NES classic
To begin with, this game has a very simple control scheme that anyone can figure out very quickly. There are two punches, left and right, which can both be launched towards the torso or face and an uppercut. You character, called "Little" Mac, can dodge from left to right and can also block.
The second thing you notice about punch-out is that for NES graphics, this game looks very good. There are no grainy textures or unclear sprites which usually cause debates with friends as to what it actually is (an NES tradition). The game has simple, clear and cartoon-ish graphics that stand the test of time very well. Sound is somewhat limited, as there is only one musical track and most punches and hits sound the same but matches the cartoon-ish feel of the game. Some sounds are just weird (when Don Flamingo gets knocked out for example) but remain in your memory forever.
After winning a few bouts, you realize that this game literally oozes personality from every character. The cast of boxers that you encounter is memorable and is one of the most unique to this day. The characters are a bit stereotyped in some cases but it's all in good humor.
There are three different classes to fight through which contain between three and six fighters. Anyone should be able to complete the first, as the challenge only really starts at the second and gets very hard by the third. If you manage to beat all those fighters who'll require you to learn all their moves and patterns to master them, you'll then be faced by Iron Mike himself and beating him is an almost impossible task that takes practice and patience. The sense of accomplishment and satisfaction is almost comparable to what you would feel had you knocked out the real Mike Tyson back in '89 (Yes, he is that hard...).
In conclusion, MTPO is a great game everyone should play and ideally own and a timeless classic that you can always come back to once you fall in love with it.