Pretty good.
New additions in this game is the "Noise change system." It allows Megaman to change into 11 different forms, and 1 exclusive form only available in its respective version.
The battle system is pretty much the same. Your limited to 3 panels to move on, choose your cards and lock on, and attack the enemy while dodging their attacks. But now there is also a "Noise Gauge." It increases when you use cards on the opponent, and different effects take place depending how high the percent is.
There aren't many version differences other than different Giga cards and the Finalize form. Noises are supposedly more common in certain versions, so if you want to get a certain Noise without going crazy, be sure to pick the right one.
The multiplayer is great, you get to become "brothers" with other people through Wi-fi by friend codes. Having brothers allows for a Multi Noise change which is kinda cool.
Downside is that on Wi-fi, theres a load of hackers and sore losers, which can greatly affect your online play.
This game is a pretty big improvement from the previous 2, which is good. And with a bunch of post-game stuff to do, this game will keep you occupied for a week or two.