Though it offers a plethora of fun new ideas, in the end, Mega Man ZX feels a bit too watered down.
Mega Man ZX isn't a bad game by any stretch, but it is medicore and rather pedestrian, which is particularly disappointing given that it tried to introduce a number of fresh elements into the tried-and-true Mega Man series. Principal of these new gameplay elements is the overworld, which feels about as functional and necessary as that in the first Sonic Adventure game: not at all. You can explore the world of MMZX at your leisure in-between blasting enemies and taking on missions, but outside of a few secrets and opportunities to fight optional enemies, there's simply no reason to do so. The world is sparsely furnished with NPCs who will spout out a few lines of text like in any RPG title, but never is what they have to say useful or entertaining. Yes, occasionally they'll assign you a mission that wouldn't have presented itself otherwise, but these missions also feel tacked on and like a waste of time. Clearly, Capcom was attempting to emulate the worlds of Castlevania, but exploration simply does not lend itself well to this game and simply fills time that should've been used to destroy more enemies in more missions. I also found myself getting quite lost in this overworld when actually trying to get to a mission location, and it was then that it became crystal clear to me that, yes, this overworld was included simply to pad the game's extremely short length. It's a shame, really, especially after how much better the worlds in the "Zero" series on GBA worked.
Outside of that unfortunate inclusion, however, the various innovations in Mega Man ZX world rather well, though aren't without their respective flaws. The most obvious and widely advertised of these innovations is the "Model" system, which basically has your character changing into entirely different settings that are accumulated as bosses are defeated as opposed to merely switching weapons, as has been the norm in previous Mega Man titles. There's a ninja-esque model (model P) that can hang off of ledges and throw thousands of shurikan at high speeds (it's my personal favorite), a model that functions well underwater, a model that allows for mid-air boosting and dual saber attacks, a model that has two gigantic arm buster cannons, and the plain vanilla "Z" model that is basically the Zero of games past. Perhaps this isn't typical, but I found that the only models worth using were the ninja and air dashing ones. The water model was useful only in the very brief instances where a level actually had water that needed to be traversed, and the buster cannon model was, quite simply, weak and slow in comparison to the others. Each model and transformation gives off an entertainingly tacky "Power Rangers" vibe, which I think was intentional. So again, we have a decent idea that was simply broken in its execution, which is too bad because the art for each model was pretty bad ass. What else? At the game's onset, you can choose to play as Vent, a male protagonist, or Aile, the female. It's a completely innocuous aesthetic preference which character you choose, because both follow the exact same storyline and get the same models, though their sprites look a bit different when compared. It was a rather frivolous thing to include, though Capcom certainly can't be faulted for doing so.
Speaking of storylines, Mega Man ZX's is rather terrible, though it's basically par for the course in a Mega Man game. A big bad man who's so discreetly named "Serpent" is attempting to collect the bio-metals so that he can form the massive Model W and take over the world, more or less. You, as either Vent or Aile, join a rebel group that fights against him. A close friend is killed and some horrendously bland scripting fleshes out the rest of the all too typical story. Oh, and there are some short though nifty anime sequences along the way, further instilling the "This is the love child of Mega Man and Fox Box" vibe. Huzzah!
If you're looking for some Mega Man action on the go, you'd be better off playing one of the three Zero titles on GBA, which could also be used in your DS, or better yet, playing Mega Man: Powered Up on the Sony PSP. Again, it's not that Mega Man ZX is bad, it simply isn't as good as it should've been.