If I had one complaint about this game, it would only be that it's too short. However I'll also say that the Continue+ mode, Endless mode and several difficulty levels mitigates this as well as it can.
I usually judge a game based on 5 criteria, listed in order of important: Gameplay, Controls, Replayability, Sound & Graphics.
Gameplay: As many people before me have said this is definitely an homage to Punch Out from the slew of eccentric and quirky characters to the first person boxing mode. The gameplay though is very tight. The AI doesn't feel like it cheats, there's no psuedo difficulty or anything of that nature. The AI learns from your patterns and which attacks you're using most and will guard against it in the current and next round, forcing players to avoid a specific pattern or style of play.
Controls: You can rock the keyboard or you can rock a gamepad, fully customizable and absolutely no complaints here. Controls are such an important part of a game, they can make a game or break it and while you might think it's standard practice to allow customizable controls, it's the few games that don't that stand out like a sore thumb. Thankfully this game isn't one of them.
Replayability: As long as you're in the mood for the game, there's content there to play. From exhibition missions to completing/replaying the story, or trying your hand at endless mode. The only thing I can foresee ruining this is becoming bored with the game itself which will happen, as with most games, eventually.
Sound: The sound effects are definitely on point and the music is about what you'd expect: standard background music that you actually hear for a few minutes and then start tuning out after that.
Graphics: The graphics for this game are great. They're not super polished high end but for what they are they look awesome and I had a ball seeing new characters and the blending of art for story and 3d in the ring.
Overall on a scale of 0-10, I'd give this game an 8.