Let's start with the beginning...the opening and ending songs (Japanese version) were great the entire way through from what I can remember. Even the first English dub opening is appropriate for the series and catchy, and is actually better than the dubbing job, itself.
I suggest, if you can, to watch it subbed; subtract 1-2 points from Overall and about 10 points from Sound for the English dub version, because it's that awful.
If your favorite character turns out to be Judai (Jaden), increase the Overall rating by 2-3 points; the show is extremely Judai-centric. If your favorite character doesn't turn out to be Judai, decrease the Overall by 1-2 points since you probably won't enjoy it as much.
The anime, itself, started off strong with a Jounouchi (Joey)-like protagonist. Unlike its predecessor, GX features your typical headstrong, hot blooded idiot of a main character with a perpetual cheerful attitude and a natural skill for the game.
However, the characters are extremely flat, especially the protagonist up until the third season where he changes too drastically and suddenly.
GX, like its predecessor, has the WORST assortment of side-kicks in terms of skill level and screen time. Shou (Syrus) and Hayato (Chumley) are jokes for the majority of the first season, then eventually fade into the background halfway through the second season (Hayato vanishes completely). Shou remains as a supporting character throughout the series (with a couple episodes involving him as more than a cheer squad), but it would have been nice to see him and some of the others get a larger role.
The third and fourth seasons also completely killed the show. The third season brought in new characters that all but replaced some of the characters we grew to love in the first two seasons. The fourth season, with the exception of a couple episodes, was actually painful to watch until the final two episodes.
If you like an anime packed full of cliches where it's the main protagonist that handles 99.9% of everything that happens, and everyone else winds up as a cheer squad, then you'll like this anime. It has the typical shounen protagonist, as well as just about every shounen cliche you can find.
If you want an anime where the lead characters (plural, not just the main protagonist) grow and continue to shine and work together through the entire series in every season, and the plot is interesting, engaging, and slightly mysterious, then this anime is not for you.