Me and three friends played Steel Diver when it came out, and as hounds for co-op, I can safely say that this game kind of sucks, though it wasn't too bad with two or maybe even just one player.
The combat is fast, chaotic, and pure button-mashing, FILLED with enemies that blow up and hurt everyone if you don't defeat them a certain way (which will happen constantly in multiplayer) and pits that will hurt you again and again until you get out of them. The problem is, when playing with multiple people, there is simply too much going on, too much speed, and too little for the camera to focus on to be able to properly play it. I can't tell you how many times me or a friend would get stuck at the bottom of the screen getting hurt over and over until our friends made it high enough for us to reach some kind of pity-platform.
You also get experience individually for each kill, which breeds a sort of competitive mindset which really sucks when some of the characters are really meant for a more supporting role; by the time we stopped playing, the red "main character" guy was about 4 levels ahead of everyone. The combat is basically just holding forward and pressing X a lot, so with multiple players being mostly concerned with just getting the most experience for themselves, you just look like a swarm of locusts. There are even some hidden treasures in the levels, but why try to get those when your friend over there is leveling past you as you search?
The story doesn't need to be good, but I just want to say that it was one of the most insane sentences (Maybe totally a paragraph once you've beaten it, though saying "paragraph" implies that it's a sort of cohesive thought) of "story" and "dialogue" I've ever seen in a game. It felt like I was playing a very poorly subtitled import.
Eventually two of us got bored and frustrated, so they quit, and my brother and I continued playing with just 2, which we found to be a superior playing experience, BUT it was no longer the party-platformer it both set out to be and advertised itself as.