Gave it my first try yesterday just to see how I liked it. I was rushing it a bit and wasn't doing so well, and stopped playing midway in level 1.
Today I gave it another go, this time I wasn't rushing, and was having a lot more fun. Unfortunately, I got most of the way through level 1 (I had just recieved the location for the portal code) and the game locked up completely out of nowhere. No explosions, nothing out of the ordinary, just jumping up to a platform above me and it froze while the music kept playing. After a minute or two the music stopped and all I could do was turn off my DS. Really disappointing to see something like this happen in a game that won't even let you save mid-level.
My other gripes would be the game is "NES hard" as in, most of the difficulty comes from primitive design styles like:
-you can't crouch and shoot left/right
-enemy respawn seems somewhat unpredictable
-enemies placed where you can't get a good shot angle
-enemies can shoot through some walls that you can't shoot through
-if I understand correctly, you have 3 lives with which to finish the game or you start over from level 1
Overall I like it, except for the lockup thing. Hopefully that's a rare occurrence, because so far that's the only thing making me regret the purchase.