It's still playable, but Metal Gear Solid is one of those games where the story really overshadows the gameplay and you probably won't enjoy it nearly as much as if you had played the rest of the series. Plus, it would pretty much ruin the other games with nasty spoilers. :P My advice to you would be to go back and play Metal Gear Solid 1-3 first. (The first two pre-Solid Metal Gears aren't really necessary because they have pretty simple stories, and neither is Portable Ops because it just isn't referenced much, if at all.)