One of the things that bugged me the most about Portal 2 was the lack of challenging puzzles in the vein of the late test chambers in Portal 1. Part of this may be due to Portal 2's many new testing elements, which reduced most of its chambers to tutorials. Portal Stories: Mel only introduces one element (death fizzlers), and it's fairly incidental, only being used in a handful of puzzles. Because of this, Portal Stories: Mel can get away with focusing entirely on challenging puzzles, and boy are they a doozy! These tests will challenge any veteran Portal player, yet are always well-designed rather than creating fake difficulty from hidden portal placements.
Meanwhile, the storyline is surprisingly complex for a fanmade mod. Mel and Virgil's attempts to thwart AEGIS aare countered at every turn, and the eventual finale to the conflict is adrenaline-pumping and satisfying.
The visuals and sound are astounding as well. Aside from the great Hammer-fu done with existing assets from Portal 2, many impressively well detailed new assets exist as well. A highlight is the tacky 70s portal gun Mel acquires at the beginning of Chapter 2. The newly composed music is on par with Portal 2's fantastic soundtrack, while carving out its own style as well. I'd say the great music was instrumental to keeping me from going mad during the game's trickier puzzles.
Overall, a fantastic game, and it's free! If you played Portal 2, and even if you thought its puzzles were too simplistic, I highly recommend giving this a try.