Could have gone far with some minor tweaks.
After making it past the abnormally large loading bar (considering the lack of exploration in the game), the first thing you'll notice is that it keeps the tradition layout a space invaders game. Alien creatures (of which there is quite the variety) stand at the top of the screen, while your chosen character (from a selection of three) stands at the bottom. The enemies then walk, fly crawl or slither towards you, shooting at you as they do. This is where the main game play changes have occurred. Simply side-stepping that amount of bullets would be difficult, and in some cases impossible, so a roll ability as been included to help dodge attacks. You also have a number of special attack points, which can be used to either throw a grenade (at the cost of one point) or a character-specific special move (at the cost of two points). Within minutes you'll have the rolling and specials down to a fine art, and be able to take anything the game throws at you. Until the end of level bosses.
No compromises have been made in ensuring that the bosses all appear as large and powerful as can be, and each has a selection of unique attacks at there disposal to give you a challenge. Which leads me to the first major complaint. Unlimited continues. They have wrecked many a game before, and this is no exception. With no penalty for dying, there is no longer the tension as you see your life bar drain away and, more importantly, completing the game offers no sense of achievement. If there was a set number of continues, the game would be a serious challenge, invoking bragging rights among friends and the constant need to finish it. But as it is, you'll finish it within hours, leaving little reason for it to be picked up again.
To try and make up for this, there is a survival mode, offering no continues and a two player option. Two player has many faults, the main one being that, as the camera moves from side to side to show the width of the area (it being about one and a half times the length of the screen), the only way to keep both players in-shot is to create an imaginary tether between the two, resulting in being un-able to shoot an enemy thanks to the positioning of the other player, and all to often getting hit by a shot because you can't move out of its way.
All that said, Space Invaders: Invasion Day is truly a great game and for its price tag, you can't really complain about the small problems that spoil what could otherwise have been a cult-hit.