The game is a mess and nothing has changed, best left for fans only.
The game hasn't improved at all and is essentially the same as X5, just with a few added extras.
Story: 3 weeks have passed since the space colony Eurasia crashed and nearly destroyed earth.
The debris combined with the maverick virus surrounding the atmosphere caused many reploids to malfunction and made it almost impossible for humans to survive on the surface, these events were known as the Earth Crisis.
Humans had to take refuge underground while remaining reploids and maverick hunters dedicated themselves to rebuilding the planet.
A few days later a giant mechanaloid went on a rampage and X was dispatched to stop it. X managed to disable the mechanaloid but at that moment a mysterious figure appeared and completely destroyed the mechanaloid before it disappeared again, then another minute later a large reploid named High Max arrived to investigate the area, X engaged High Max in combat but ultimately lost.
A few moments later a world wide announcement was made by a reploid named Isoc.
Isoc announced that the world was under the threat of a viral program called the Nightmare and was supposedly generated by the remains of the missing maverick hunter Zero, and as a response sent 8 investigators to scout the areas for any clues on the nightmare.
X, taking the accusation placed on Zero despite having sacrificed his life to save the earth personally, decided to uncover the truth behind this "Nightmare" and learn what Isoc and High Max were really up to.
Game design: The environments look decent, though it would have been nice to see some background animation than just plain images scrolling for one side to the next, the game has also been sped up a bit.
The boss sprites this time around aren't so great, at this stage Capcom should have considered adding better and more detailed sprites and should have added better animations.
The story is once again presented in image slides but this time with spoken dialogues, however Capcom didn't bother to fork over the cash for proper translations and as a result you will find lots of grammar and spelling mistakes.
There will once again be multiple endings depending on how you progress and who you play as.
The stages aren't very dynamic and for most parts are poorly designed, because there will be places where you will get permanently stuck just because you didn't have the right equipments and unless you have already completed the stage you will end up killing yourself again and again until you can get back to the stage select, this can make the game very irritating.
Alia will once again help you through stages by offering advise, this time though she will signal you and if you want to hear what she has to say then press the select button otherwise you can just go on.
A new feature would be the Nightmare system, each stage can affect the other so once you can complete a stage others stages will be highlighted in red, indicating that its affected, which can make completing them easier or harder (mostly harder).
You can also rescue people along the way, but unlike in X5 rescuing these people will have its benefit, some will carry items with them that you can equip later on in the stage select screen, however you will have to rescue those people before they are consumed by the nightmare, otherwise they will either go missing or dead and you won't be able to get a certain item through out the entire game.
Game play: You start out as X and can later unlock Zero.
X's game play is the same as usual except that this time he wields the Z-saber, however his swing are slow and aren't as affective as Zero's, so you will find yourself using the X-buster most of the time.
X will start with the Falcon armor from X5, the armor's functions aren't the same though, the armor can't fly and the buster won't penetrate through walls, but you can now charge special weapons and can still use the Giga Attack.
Throughout the game you will gain 2 new armors, The Blade Armor and The Shadow Armor. You will have to collect all 4 parts for each in order to use them.
There are 4 parts for the Blade Armor, The head module will reduce special weapons energy consumption, the arm module will convert the X-buster into the X-Saber, the X-Saber can be charged and released to make cuts that matches that of Zero's final blow, it also comes with a charge shot that sometimes stick on the enemy for additional damage and can also charge special weapons. The chest module will increase X's defense and will also give X a new Giga Attack and the leg module will allow X to air dash at any direction.
The Shadow Armor will also have 4 parts, The arm module will also Convert the X-buster into an X-saber but it can perform a charge saber attack that's much powerful than the Blade armor (more like a Katana cut) but won't have a charge shot, the X-saber can also shoot scatter shots for normal shots (something like shurikens) However you won't be able to use special weapons. The chest module will increase defense and will give X a new Giga attack, the head module will allow X to make faster swings and the leg module will allow X to cling on ceilings and walk on spiky terrains.
Zero will also be playable, this time with a new saber and a new fighting style, he will also come with an air dash and double jump with a spin attack at default.
After defeating bosses X will gain new weapons and Zero will learn new techniques.
The nightmare system also plays a role here, through out stages you will encounter Nightmares that will drop nightmare souls once destroyed, these nightmare souls can be used to upgrade the hunter ranks of either X and Zero.
The higher the ranks the more items you can equip.
Sound: The music is as good as ever and there is a BGM test for it in the option screen, sound affects are EXACTLY like in X5 and as I mentioned before there are spoken dialogues but only Japanese with English subs.
The game does have a lot of content and has really fun and exciting boss fights but perhaps that's all there is to it.
To put it simply Megaman X6 is a completely rushed and unfinished product.
The poor level designs combined with the terrible presentation will turn many people off and will irritate fans.
The way the story was presented in image slides (in favor of removing AMV cut scenes like in X4) in X5 was already bad, but the terrible English translation as well as not providing English voice casts for the English version is beyond unforgivable, let alone this is a PS1 game that could have handled a lot more (I mean look at Final Fantasy, in 1 disk they managed to put in 10 full motion videos and this game doesn't even have 1)
Clearly Capcom has not put any effort in the game and were only after the money.
Overall fans of the series and side-scroller should get this since it still manages to offer some fun 2D side scrolling action (though in an irritating way) but be prepare for some disappointment, others should stay away.