It's obvious that Ikari Warriors was developed for the arcade. It's that difficult. Although I find myself making it further and further each time I play, this beast must have raked in numerous dump trucks of quarters over the years. You play, you die, then all you want to do is play again. You never really get as far as you wanted.
As a 1 player, it's nearly impossible to beat, but add in the help of a friend and usually we can beat the first level or so. There's only four levels, but the last time I got all the way to the fourth (by using the re-spawn code) it took me 3 hours. WOW.
Although it's still fun to play with the re-spawn code, consider yourself warned; This is one unbelievably hard game.