XIII is completely underrated, but is an awesome shooter with exciting multiplayer and unique storyline.
The graphics in XIII are very different than most games. It attempts to represent an average comic book. Almost all sounds are seen just as well as heard, including the "tap tap tap" of people walking, the "BAM" of rockets crashing, and the "nooooo" of dying foes. This gets quite amusing sometimes. The graphics are very cell shaded, meaning that the shadows are easy to find and are only one shade different than whatever is casting them. With non-cell shaded graphics, shadows go through many shades before reaching there darkest point, which is an attempt to mimic real life. The greatest benefit of cell-shading (at least for me) was that I could play XIII for hours and my eyes would never hurt.
The music and sound effects in XIII creates a great environment for the stealthy and unique plot. The music includes some cool jazzy grooves as you sneak up on foes and snap their necks. The snap of the neck is just as good. The "tat tat tat" of gunfire is great, but the only problem is that when an enemeny is firing the same gun as you, the gun is making a different sound as when you are firing it.
The control in XIII is basic First Person Shooter controls. The shoulder buttons control firing, jumping, and crouching, the analog sticks control movement, etc.
The greatest part of XIII is the storyline. A man is blamed for killing the president, and is being hunted, but he doesn't even know who he is or what's going on. Many characters join the story, and by the end of the game everything finally comes together.
It's too bad not too many people played XIII, because for big fans of it, its unfortunate that the "Too be continued..." at the end of it might not come true.