I'm sure if you read some of those reviews, you'll see why people like it so much!
As Jammer said
"Before and After" is yet another Trek breed of the Time Manipulation Paradigm, but I'm not about to hold that against the show. This is a weird, effective story that uses its plot machinations as a springboard for some thoroughly enjoyable mini-stories. The episode is paced at a brisk speed and filled with sub-stories that contain humor and fascination, venturing into both hope and despair.
The relentless jumping through time makes the story interesting and fast-paced--as does the way Kes and the crew come to understand the nature of the mystery--but what really makes "Before and After" compelling are the "what if" implications. The story paints us one possible future of the starship Voyager, and it's in these details that the show gets truly inventive and entertaining.
I can't really put it better myself