I figure since I sat down and actually finished the game, I'd talk about my final thoughts before it goes back on the shelf for quite a while. Mighty Final Fight is a 100% no frills brawler, more in the arcade style than most of the brawlers I remember on the NES. It really does feel like an entry in the Final Fight series, for better or for worse.
First off, it's definitely one of the best looking and sounding non-Nintendo games on the NES. Everything is drawn clearly with great animations and there's minimal flicker. The soundtrack is vintage NES Capcom. In fact, it was composed by two of the folks who did Mega Man X's tunes.
More importantly, the game itself actually plays pretty well. You have a selection of three characters: Cody, Guy and Haggar. Disappointingly this is single player only, so solo artists or turn-takers only. Anyway, your standard brawler fighting is what you'll get here. Punch with one button, jump with the other. Jump kick using both. However, it turns out there's a little bit more depth to the combat than my previous post stated. I looked up the controls to see if I was missing anything and Cody and Guy have a knee-drop maneuver that Haggar (the character I was playing as) does not. This actually allows you to come in on enemies hard and fast, opening up a lane for attack more safely than you can without it. It changed my opinion of the fighting system quite a bit, and the game got a lot more fun once I knew about it. Also, Haggar feels much less useful because of this and his lack of speed, but at least there are two other characters to play.
Still, the game isn't without some of the same hangups many brawlers before and after suffer from. While I've come around on the bosses and think the fights are actually pretty fair, sometimes enemies in the game, boss or not, are just going to hit you. No matter what. It can feel a tad unfair in that regard. The levels are pretty much just walk forward, kill half-of-to-a-dozen enemies, then move on to the next segment, all the way through the game. Sure, there are barrel-breaking bonus sections, but those are brief and don't really feel substantial enough to break things up.
And then there's how the experience works. If you die, you restart the stage at the same level you were at, so if you entered level 3 with a level 3 character, but level up to 4 during the stage and die, your character's level goes back to 3. But you start with 0 XP, no matter what you originally entered with. Once you get up to stage 3, the board won't provide enough XP to make it up a level, feeling like you're capped at whatever level which you had to continue unless you can do two in a row without using a continue. This, frankly, makes leveling up feel impossible. It rewards players who know what they're doing, but it can definitely cause newer players to feel even more hamstrung than they might already feel. I finished the game maxed out at level 4 of 6 possible tiers, so it's not the end of the world, but it's still kind of a crappy system.
I'd say Mighty Final Fight is a little rough around the edges but probably worth a play if you can get your hands on it, especially for brawler enthusiasts. The US cart is so prohibitively expensive, and there aren't many games I would ever recommend spending that kind of money on, but the much cheaper Famicom cart did the job just fine.
There are truly few great beat em ups.
RCR. Double Dragon II (Fami/NES version). Streets of Rage 2. Turtles III and IV. And I personally love Return of Double Dragon, though it has a weak framerate.
I agree that there are few truly great beat 'em ups. You may have left a few off though, with Guardian Heroes being a huge favorite and some of the later arcade brawlers my friends swear by like Armored Warriors, Aliens vs. Predator and Battle Circuit, but even then the ranks are thin.