MERCS is the belated follow-up to Capcom's Commando, and while it improves aesthetically it fails to fix the one really big problem with the gameplay.
The problem with Commando was that it was essentially a rail shooter trying to have action-adventure controls, and it wound up being a clumsy combination. The one thing that would have completely redeemed the gameplay would have been the addition of a button that let you hold your aim in a fixed position while still being able to move freely; I thought at first that maybe 1986 was too far back into Prehistory for game programmers to be able to handle such a concept, but by the time MERCS rolled into arcades in 1991, still minus this basic feature, there really wasn't any explanation left other than Capcom just didn't care.
The enemies, of course, can strafe like madmen, meaning that even the most basic of the grunt hordes that you blow away is technically more competent than your character is. The only thing that stops them from making a total laughingstock out of you is that, somehow, their bullets travel at about the speed that a child throws a baseball, so you can pretty much outrun and step around them while you mow them down with your uber-guns packing infinite ammo and ridiculous fields of fire. Your character also has a health bar now, which allows him to absorb even a cruise missile or two with only a hearty "Huuuuuuuuch!" in response, and a set of screen-clearing explosive devices that are immensely more useful than Commando's tiny grenades.
The story seems to be that the President of the U.S., while touring Central America, was captured by one of those rebel factions that Rage Against The Machine's lead singer left the band to join. Due to "diplomatic reasons", the for reals military can't be sent in (lol wut), so instead this covert team called the MERCS (I'm still not clear on whether they are actual mercenaries or just an army squad with a confusing nickname) is called in to single-handedly (or up to three-sets-of-handedly, depending on what version of the machine you have) blow the hell out of the rebels and rescue the Prez. For some reason, all the Mercs look more like gay porn stars than commandos, but I guess that's neither here nor there.
This leads to five levels that were fairly graphically impressive for the time, along with pretty good animation and asplosions for the period. The music and sound are unremarkable, except for the fact that some of the music (or at least the main motifs) seems to have been recycled from Magic Sword (which came out about the same time as this.) The game is an OK romp, and picking up where you died with each quarter makes it much more manageable than Commando, but the play control is still frustrating and on the whole it won't be likely to draw you back for another round once you've tired of it initially.
* Gameplay Video