Nothing exceptional, but not terrible either. Oh yeah, and boobs!
The graphics are decent, though still not on par with Half Life 2, both of which are built on the Source engine. The polish is lacking... some clipping on character meshes and low-detail textures to be found here and there.
The dynamic difficulty system is a good idea, they just couldn't get it to work quite right. Game difficulty increased sharply towards the end, pitting one against large numbers of minigunners and fliers on 3 foot-wide ledges no less. In one memorable sequence, I must've died no less than 15 times before passing it. Despite the more frequent deaths, the difficulty system never seemed to let up. A patch or two should iron those glitches out.
Overall, the AI was erratic. Sometimes I caught enemies staring at walls with their backs turned, apparently oblivious to all the gunfire. There was also a scripting bug which required a console command to move the game along - nothing that can't be quickly fixed, but that should have easily been caught by QA/testers.
The character designs were rather mundane and lacked imagination. In fact, the mutants looked remarkably similar to those found in HL 2. There aren't many types of enemies either - basically 3 human types and 4 mutant types, one of those being a boss.
Jessica is cute and Elexis has two bowls of Jello. Barring that, this game has nothing special to set itself apart from any other FPS.