I wouldn't say the graphics are amazing, but they're certainly good. And the alpha is a very old build, so I'm sure they've improved significantly.
What impressed me is the SCALE of the place. The indoor areas are pretty detailed, and the outdoor areas are HUGE. Huge as in Serious Sam. The engine copes seamlessly going from a detailed indoor area to a vast plains-like outdoor landscape.
Plus, the level design in the alpha is rudimentary at best, but it's already creepy as hell. It looks like STALKER's going to be as good as they promised!