Here are some impressions from friendly importer and NeoGAF user, Dash Kappei!
As expected, the game feel a lot more like Metal Slug DS than the Metal Slug number 7 it's been titled by SNK PlaYmore.
It's easily the best handheld Metal Slug out there, but maybe playing Anthology on PSP from a memory stick (no loading) beats it, never tried it so I won't compare... but I was a lot more confortable with DS d-pad for the game then PSP's one, that's for sure but it all comes down to personal preferences.
Graphically there's a lot of good to the game and some fail.
The main gripe it's due to the machine hardware much more than a lacking effort by the developer: the screen resolution makes for too tiny characters and a lot of jaggies... but it's still all there and Metal Slug's charm is definitely retained.
For the first 5 levels I've played I'd say slowdowns are almost non-existent, framerate could be smoother though.
Difficulty wise there's definitely a challenge here (3 difficulty levels, dunno about unlockables) and I applaud the choice to give the player only a limited amount of continues (9 for beginner, 4 for Normal and just 2 for Hard) and you have 6 characters to choose from (Marco, Eri, Tarma, Clark, Ralf and Fio) along the usual dual slot for selectable guns like the latter games in the franchise.
On the way to unlocking the first 5 levels you'll be able to use any one of them as the starting point for your own adventure, minus the last 2 levels, so there's a total of 7 levels.
Visually I don't like the choice, a bit like Slug 6, to go with prerendered background since DS limited capabilitis make for an even poorer color palette and grainy graphics, but under a gameplay point of view it definitely help DS's screen to make enemies and bullets easy to spot (so it makes a lot more sense here than for the arcade chapters).
The game doesn't use any of DS unique capabilities but I'm not complaining, leave this action classic alone, even if I have to admit a more clever use of the 2nd screen (showing a useless minimap) would've been appreciated and, dare I say, expected.
Apart from that it's the usual godly Slug's fanfair with humour, great 2D art and animations, motherfuggin' big bosses and all that... I'm happy the franchise graced Nintendo's hendheld and it's worth your cash if you're looking for portable Slug action and you don't have a PSP/don't like Anthology PSP or you can't get enough of Metal Slug for the get-go.