one of my favorite gba games, up there with gba port of zelda link to the past and advance wars 2.
if you are thinking of getting astro boy or contra or another 2d sidescrolling action game, don't. get this instead first. it is very challenging and no game out there allows you to fire this many bullets, (and rockets, grenades etc.). the cartoonish violence is exagerated and taken to a comical extreme in metal slug. for example there are numerous instances where enemies weilding rockets (ranging in size from 2 feet to 20 feet) litterally stab you with the rocket. soldiers throw rockets with their hands. boss war machines must be shot hundreds of times with machine guns and other awesome weapons. metal slug is over the top.
the general character of metal slug is kind of generic, but somehow it always manages to seem fresh. when the first metal slug came out it was litterally $200 just for the cart, and like $600 for the neo geo system it played on. now you can buy it for $30 for your gameboy advance (probably used) or wait for metal slug 1 gba to come out, or get super lucky like me and get a clearance copy at toys r us for $10.
metal slug advance seems kind of short, in spite of its very high level of difficulty (compared to similar games). especially since you will be foaming at the mouth with excitement when you get it, and will likely play for hours and hours until you triumph. but you definitely get your money's worth. every level is substantial. you can choose between two different characters and you can save your game. also, this version of metal slug has a card collecting game that adds another dimension and is incredibly difficult. most of the cards are hidden and in order to keep the card you need to successfully complete the mission without dying!! so far i only have cards for the first and second missions. supposedly there is a whole nother 6th mission, the "dungeon mission", but you need to find and keep the "dungeon card" to get to it. the dungeon card is on the 5th mission, which is VERY difficult to win without dying.
so basically if you like video games you will like metal slug advance. buy it.
if you are still reading here is a little cheat. warning: spoiler. ? location of dungeon card: go as far as you can on the fifth mission with the aerial slug so that you don't have to parachute. then when you procede through the next couple areas and get a choice between two routes, take the UPPER route (not the lower). the second time you take the upper route you will enter a room with a tank and a hostage. in the background is a big storage shelf with 4 quadrants. shoot up into the UPPER LEFT quadrant until you get the dungeon card. there are two cards hidden there. one is the dungeon. now all you have to do is make it through the mission and the boss without dying. unfortunately that is impossable. sorry.