This was one of the first games i bought for Nintendo DS, and it is still one of my favorites. When i saw there was a reviewing competition going on, i thought i might as well review it, mostly because i was bored. I haven't written a review in ages and English is not my first language, so hopefully you will understand most of what i'm saying.
The game has 3 difficulty modes, Beginner, Normal and Expert. Only Beginner and Normal are available at the start of the game. Expert mode is unlocked by beating Normal mode. Beginner mode lowers the damage you take from enemies, makes bosses take more damage and removes dying from touching spikes or falling into a bottomless pit (Biometal A carries you out of them for a short time). Normal mode is your normal Megaman game difficulty. Expert mode is the exact opposite of Beginner mode, the bosses also have a better AI and use their stronger moves more often.
The game also has voice acting, something Megaman ZX didn't have. However they can sound weird because the voices end when the onscreen message ends, so they can just end in the middle of a sentence and continue when you press A.
You can pick to play as either the boy reploid Grey, who has amnesia or the girl Hunter Ashe. Their first stage is different and their story is a bit different. Greys first stage is the Mysterious Lab, and Ashes first level is Raider Pursuit. Both stages are pretty much tutorial levels, and they both fight a similar boss at the end of the stage, but he uses different attacks. Grey runs into Pandora at the start of Mysterious Lab, while Ashe runs into Prometheus at the end of her stage. After that their story is mostly similar, with Greys story being focused more on regaining his memory, with Ashes story being focused on helping the Legion. They go through the same stages and the only major difference is that Grey encounters Allie (from Megaman ZX) and Ashe encounters Vent (from Megaman ZX). The story is interesting and has a lot of plot twists, though the main villain was pretty obvious because of his name (but not all the other plot twists involving him). I also love how both Grey and Ashe have connections to the villain.
Gameplay wise, the game is almost the same as Megaman ZX, with a major exception being that you can transform into Pseudoroids and Megaman bosses by using A-Trans. Sadly most Pseudoroid bossses aren't that useful (Chronoforce and Rospark come to mind). Megaman bosses however are all really strong and if you want to make the stages easier for yourself, you should think about using them. Note that Grey and Ashe have similar attacks in Megaman forms, but they are not the same. You can also collect medals in this game by beating bosses in certain ways, most of them need you to accept a mission to have a rematch with them. I never bothered to collect them all because some are hard to do and the reward for getting them is simply a classic Megaman sprite Model A that also plays like classic Megaman.
Unlike in Megaman Zx, stages are way easier to find in this game. If you got a red card, you look for a red door, only exception to this is the Quarry stage where you have to use Queenbee A-Trans to reveal an entry to it. My only complaint with some stages would be that they pretty much force you to use a specific A-Trans, the Scrapyard stage was pretty much designed for Model P for example, and while i love Model P, i can see that being an issue with other players.
The music of this game is pretty good. I only don't like 2 or 3 tracks from the entire game, otherwise i like them all. However while the boss music (Slam Down) is good, you will get tired of it after listening to it sooner or later since 13, if not more bosses use it.
I give this game an overall score of 8.5/10. This is because some stages force you to play a specific A-Trans, and some Pseudoroids aren't as useful as they could be, and Model L is only useful in the 2 water stages that exist once you get it.