In a word: Addicting
Shadow Complex is a title found on XBox Live Arcade, that, realistically, carries more content and polish than many full retail games. This modern day "Metroidvania" style shooter has all the aspects of any good side scrolling action adventure, and brings a new level of production quality to low priced games.
As you'd expect, this is a game about exploration and upgrades. Those 2 concepts can really sum up the whole driving force behind Shadow Complex, but what is most important is that every moment of this game is pure fun. From the moment you enter the complex, carrying nothing but a flashlight, to the moment you complete the game as a fully armored missle launching, jetpack wearing army of one you will experience nothing but pure enjoyment and addictiveness. Addicting is probably thow I would describe this game in a single word.
The weapons and gadgets you are able to find as you make your way through the complex all offer something new and fresh to the gameplay, as well as open up new areas for you to explore. Finding as many upgrades as you can will inevitably make the games boss fights much more manageable, as well as bring you closer to that 100% collection status you'll invariably yearn for. The complex itself is a sprawling underground base with many varied areas and environments, as well as enemies and hazards.
There are some other features besides the story mode, such as the Proving Grounds mode. This is a series of levels designed around a single, more difficult objective, and essentially is there for those who want to prove themselves by getting the highest scores on the online leaderboards.
Overall, while this isn't a full retail game, it carries much more than its price would suggest, and I firmly believe that it is a much more complete game than many full priced games. This is a must have for really any gamer, but most certainly for anyone who enjoys solid side scrolling adventures such as Metroid or Castlevania.