Can't blame ya for not trusting magazine reviews. That's like flipping a coin. A very biased, misleading coin.
The GBA Metal Slug consists of five missions and one secret mission. The feel of the game is similar to that of the NGPC Metal Slugs, however the game engine is much more like the arcade Metal Slugs. The missions are short, and if you die (you have approximately three or four hit points) you can continue from the begging of the room or area you were in. You will die more than you'd expect because the screen is much smaller, allowing the enemy surprise attacks and you less room for dodging. But there is a reason to beat the levels without using this cheap continue feature. Hidden throughout the levels are a hundred collectible cards, some of which unlock new game features. If you die after you get a card, you lose it, so the only way to unlock everything is to find the hidden cards, get to the end of the level and beat the boss without dying. Which is no easy task considering the huge amount of HP the bosses have, not to mention some cheap pitfalls..
So, I've mostly listed a bunch of bad things. That's because you already know the good things; it's a new Metal Slug game! And yes, they fixed all the troubles in the E3 demo. Any Metal Slug game owes it to himself to get this game, despite its numerous flaws. And if you don't trust me, you could always emulate it just to try it out and then buy it.