Blast'em fun in quick, short bursts.
As many have said here, the graphics are in keeping with the arcade originals. They are cartoony, full of individuality and a joy to behold. The sound is also very much in keeping with the original games (down to the screams of dying soldiers). Controls are simple, responsive and not to blame when you die (yay!)
If there is one sticky point with Metal Slug it's the difficulty/gameplay issue. The levels of the game are hard, though not unfairly so. Once you've learnt the best way to tackle them they become much more fun to replay - which is the attraction of old-school platform games in the first place. The levels themselves aren't excessively long if you know what you're doing. It can be incredible fun to just rip into the enemy soldiers on a level you know, beat up a few enemy bosses and then turn the game off.
Essentially, Metal Slug is a game that you'll enjoy the more you play - not necessarily while you're learning how to beat it.
The difficulty of the game is mitigated by the fact that you have infinite continues. While learning how to beat a level you can keep restarting at the beginning of the current section. Once you're learnt the level, the challenge becomes to try to get to the end of it without dying in order to keep possession of the "cards" you collect during the level (and also because of the incredible "I am a hardass" buzz the game gives you).
To sum, great looking/sounding/playing game that's hard to learn, but once you do, keeps you coming back.