Metal Slug 6 is a good arcade game, with a great old-school gameplay that doesn't feel old at all!
The story is simple, while fighting Morden's men, and the aliens (which appeared in Metal Slug 2, X and 3) a NEW alien race appears, and beats the hell out of the soldiers, and EATS the old alien race... In desperation, a new alliance between Morden and his soldiers, the first alien race, and the Metal Slug heroes is born to fight this new evil that threatens everyone.
The action's still the same, you run through 5 stages shooting at everything on your path and trying to survive the storm of attacks and enemies thrown at you, and rescuing the hilarious prisioners on you way. But they added a bit of depth in the game, now each character has different skills and a melee attack, making each one better for a different situation, Marco's pistol's stronger then normal, and Tarma's abilities with the slugs are much better then everyone else's Eri's a grenade specialist, Fio gets more ammo for all the guns and always starts with a heavy machine gun, Ralf takes 2 shots to die (but he gets half the average ammo for his guns), and like Clark he has a special melee move... WAAAIT A MINUTE... Ralf and Clark? Yeah I said Ralf and Clark, the characters from Ikari Warriors, but more known as fighters from The King of Fighters are in this game! They received a comic look like the other characters, and kept some of their moves to kick some butt! And of course, as you expected, many new crazy looking enemies, and some new slugs (the series' trademark battle vehicles) too!
The graphics have been a little improved, and the sound's exactly what you would expect, new songs, explosions, shooting, and screams of pain are all there, just like it has always been.
The game length is about 50 minutes, but you'll most likely play it over and over again. There's also the Combat School where you have some missions to accomplish inside the original stages, and an art gallery for the fans.
Overall this is a great new sequel for the series, if you are a fan, buy it! If you are new to the series or just casual, you might rent it first, because the game length will not please everyone.