In Scrabble, the X tile is worth 8 points. But the expected value of drawing an X is more like 40 points, thanks to the five two-letter words AX, EX, XI, OX, and XU. (An ex is the guy you used to date. Xi is a Greek letter. The xu is a former Vietnamese monetary unit. But if you stop to read the definitions, you'll never be an elite Scrabbler.) If you know the trick, it's very easy to score up to six times the value of the tile.
So in casual Scrabble, the X is a bomb.
But in competitive Scrabble, each player in a typical two-player match will rack up around 350 points. This means the average play is already worth 30 points or so, and a good play, one where you play all your tiles, is worth 60 points at least. It's hard to play the X in a seven- or eight-letter word. So drawing the X is bad in championship Scrabble for the same reason it's so good in a casual game: it's good for 20-52 points, but beyond that it doesn't much reward skill.