Rexaura is a mod for Portal based on high energy pellets and several new mechanics surrounding them.
Map design: Excellent
Voices and humor: Excellent
Use of custom content: Excellent, even though there wasn't too much new. It was good that the original Portal feel was kept.
Difficulty: Hard/Expert
It was so tempting to find spoilers for some of the puzzles, but I managed to do them all without cheating. It took forever. I almost never do that for puzzle games/mods but this was well made and it was worth it.
I normally don't give 10 to puzzle-only mods but I honestly can't find any reason to complain about this. :/