Just picked it up the other day. So far, it has proven to surpass my expectations. Control is as solid as the NES original, the fighters are colorful but tough and the graphics/presentation stands as some of the best on the Wii. I was stuck on Don Flamenco (of all people - he used to be such a pushover, not so much anymore) and getting my butt beat, so I went over to exhibition mode where you have to try and meet certain challenges (i.e. on Glass Joe, "Knock him down three times and let him win - by decision" - a lot harder than it sounds). The sheer amount of depth added makes this more than just an update to an old classic.
The callbacks to the old NES game are quite amusing (the remixed music, the "pink jogging suit", Doc quickly amending his "Nintendo Fun Club" advice).
The two-player version is nice but nowhere near as fun as watching friends play and shouting advice. Essentially, each player is a variation of Little Mac. It starts off split-screen but as soon as someone has enough "juice", he turns into a hulking version of Mac that has powerful punches (but weak defense and easily stunned for those 1-2 counter punches). It's amusing enough as a novelty, but it would have been nice if one player would have been able to play one of the other boxers who tries to beat Little Mac. Doesn't matter, the two-player mode is not the biggest bullet-point on the box.
There are a few control options available, but old-school gamers will definitely make a beeline to the Classic NES scheme. You can use the Nunchuk and make the punching motions (the analog nub acts as your dodge), but again I feel that's more for novelty and not for someone looking to really exploit each fighter's timing. I don't have the balance board, but I've heard mostly negatives about using it for dodging attacks.
Punch-Out is definitely a must-own for the Wii. The Nintendo fans are finally getting a quality title after the title wave of bottom-of-the-barrel motion-control-exploiting nonsense that's become all too common.
(OFF TOPIC: Didn't want to start a new thread, but it seems they are also re-releasing the Metroid Prime trilogy to one disc in August that incorporates Wii play control for each title, as well as other improvements. MSRP - $49.99. Glad that Punch-Out won't be the last great offering for the Wii.)