Super addicting. Adding a time limit for each piece (or else it gets dropped where it happens to be) would ramp up the difficulty curve a bit and make for shorter, more intense games.
I made it through at least two dozen levels before figuring out that I could pick up objects and control the color of my portals (instead of just blue/orange/blue/etc). Found lots of ways to essentially break the game to make it that far, too.
 "Prison Planet"){kind=icon}
 "The Linear RPG"){kind=icon}
 "The Visitor: Massacre at Camp Happy"){kind=icon}
 "Quadrus"){kind=icon}
 "Tentacle Wars"){kind=icon}
 "Toss the Turtle"){kind=icon}
 "Dream Tower"){kind=icon}
 "Homerun In Berzerk Land: Berzerk Ball"){kind=icon}
 "Portal: The Flash Version"){kind=icon}
 "Creeper World Training Sim"){kind=icon}
 "Can Your Pet?"){kind=icon}
 "VVVVVV Demo"){kind=icon}
 "Balloon in a Wasteland"){kind=icon}
 "Abduction!"){kind=icon}
 "Record Tripping"){kind=icon}
 "REPLAYING :the game:"){kind=icon}