It's hard to actually critique a game as seminal as the original Metal Slug--it's one of the best side-scrolling shooters ever made. That sort of sums up the review right there. Just know that there's a reason this series is spoken of in hushed, reverent tones, and that reason starts with this game. The cartoonish, exaggerated expressions of the characters and the oddly bulbous war machines give Metal Slug a unique look. You'll see some slapstick humor too, something most genre entries don't offer! Sometimes I have lought out loud, because of the sometimes really weired animations!
In my opinion the hole MS-Series is a must! Buy it! If you own a Wii, PSP or PS2 you can buy the Metal-Slug Anthology, which contains MS 1 - 6 (including MS X)! There are no excuses, soldier! (less info)