Bangai-O, like Armada or Sonic Adventure was a first generation title, but unlike those games, Bangai-O really "looks" like a first gen title!
The gameplay is simple, and there will be many who might even find it addictive, where the player is in a 2D, side-scrolling world, flying up and down, shooting rockets and missiles at space-buildings (I'm assuming they're space buildings because they're built on the ceilings, unlike our common ground-built terrestrial housings), and running across the occasional space-boss.
Dialogue between the two main characters inside the robots is often confusing, as if they're each talking to someone else who isn't there. Even worse is when they chat with the bosses prior to their battle, these conversations are beyond confusing. It's clear that all these exchanges were meant to be funny, but they just come off like something was seriously lost in the translation. But the discourse between the characters doesn't effect game play, or course, just annoys the gamer.
The best treat the game has to offer, as far a game play, is that you can charge up your missile attack, by just holding the button, and when you're about to be hit by enemy fire, you release, and depending on how near you were to destruction, up to 400 missiles will fire, homing in on your enemies, and cluttering up the screen without any slow down. It's the closest thing Bangai-O has to being fun.
So, if a different company had made this game, say perhaps, a company that didn't know how to make shooter games, we might be able to look at it and say "nice effort, better luck next time". But Treasure, one of the most beloved companies in the world, went from Radiant Silvergun to this? I'd say its worth picking up, so you could say you have all of Treasure's shooters, if nothing else ... even the crappy one.