Gameplay: 9/10

• Personally preferred perspective (first person), +1/10.
  
• Puzzles actually requiring thought, +2/10.
  
• Fun co-op, +4/10.
  
• A fun new idea (portals). +2/10.
  

Graphics: 10/10.

• Great graphics, +8/10.
  
• Well designed maps, +2/10.
  

Bugs: N/A

• I found 0 bugs, no effect on score.
  

Altogether, 19/20 score.
19/20 equates to about...