NAM is so frustratingly difficult it just isn't possible to enjoy what the game actually has to offer.
For the record, I'm playing the version of this game called 'Napalm'. As far as I know there is no real differences between NAM and Napalm. Don't know what the story is about all that, but I don't care anyway.
First things first, the story: you're some marine named Alan "The Bear" Westmoreland (oh wow) who's a experimental killer for the CIA, all dosed up on a magical serum that acts as a "healing agent, pain killer, and stimulant all rolled into one bad-ass cocktail." Quoted directly from the manual by the way. Basically it's pretty ridiculous.
As I said in the intro, this game is running on the build engine. That said, don't expect much in the way of graphics. I still like the engine - but you really have to look at the technical side here. NAM runs on the same version of the build engine as Duke Nukem 3D. Now, I'm all for using the build engine. But the version used in NAM is outdated. This is not the case for other 1997 build games like Shadow Warrior and Blood, which use more up-to-date versions that are capable of advanced graphics and effects. I'll just sum this up without the technical stuff: It's pixelated and looks bad.
Which leads me to the actual gameplay. I found it difficult to pass the first level. I'm dead serious. I played on the easiest difficulty, too. This is one of the only occasions where the graphics actually affect how fun the game is. Due to the horribly pixelated foliage and bad AI the enemy can see you, but you can't see them. Snipers (and all enemies, actually) kill you almost instantly, mines are tiny and spread all over and you'll be pressing the quick-load button time after time. You can't see the mines - because of all of the vegetation and trees blocking your view! Supposedly you can press space just after triggering them to disable them but I never got that to work. Doesn't seem very realistic, anyway.
Now, I'm actually starting a little section here for stolen content. Many of the game's textures are taken from Duke Nukem 3D. Others are taken from Redneck Rampage and Shadow Warrior. Much of the game's code is exactly the same as Duke3D's, or if it is changed it's very minor. Some sounds are ripped from Duke. Even the one-liners the protagonist says during combat are exactly the same as the ones Duke was notorious for in Duke3D, although some new (and pretty bad ones) are added. This game is basically a Duke Nukem 3D modification, and a bad one at that. You even crack your knuckles at the beginning of every level! And the level ends either when you enter a certain area, or press a large button... that acts exactly like the nuke button from Duke.
The music and sound for the game are bad and okay. Music is average, but there's only a few tracks for the game so the same tracks will be used throughout the game OVER AND OVER AGAIN. It gets annoying. Sound effects are decent.
Another thing is the realism. There's absolutely no recoil (even for the M60). The only weapon you reload is the M16, although you do cock the shotgun after every shot but don't put the shells in. The flamethrower gibs enemies instead of lighting them on fire. You have a knife in the game, but it's pointless and when you do use it it takes quite a few hits to actually kill an enemy. You can kill an enemy with a shotgun from across a field. There's more but I won't list them. Oh, and apparently your character has three arms since you can reload the M16 and swing your knife at the same time.
There's also a multiplayer mode but I couldn't play it because no one plays this game any more, and based on Gamespot's review no one did back in the days either.
Just stay away. Stay back, I'm warning you! This thing is dangerous to the heart and soul. Okay, I was exaggerating a bit with that but I wasn't exaggerating with one thing - this game is poor. The game actually spawned a sequel, WWII GI, surprisingly. Anyways don't get it because I can't guarantee you'll have fun with it. Just get a more recent Vietnam game like Men of Valor or even Vietcong, you'll enjoy those.