When I first watched the original Yugioh, it was good, but got boring after awhile. It basically had the exact same storyline all the way through the whole story, and consisted mainly of just dueling.
Yugioh GX, however, has some different story lines, though it has a lot of dueling as well, which gets kind of old after awhile. Watch a few duels that has Jaden in it (Which most of the duels do), and you basically know his favorite strategies, and you basically memorize his whole deck, so you aren't surprised if he summons Elemental Hero Flame Wingman onto the field; one of his classic moves for a lot (I think) of the first season. Later on he for some reason, uses more of a variety of monsters in his dueling, such as Elemental Hero Mudball Man (Or something like that). The only person that uses some variety of decks is Bastion Misawa, and it's amusing when you first learn he has so many decks that somehow fits under his yellow shirt.
The series is about 8/10, until the end. I find the storyline that happens later on, (with Jaden losing himself; kind of similar with Yugi, in the original Yugioh, and the Seal of Oricalcus), if you didn't understand what I just said (well, actually I wrote this, but it's just an expression, so I won't change it), then you have to watch a lot of the episodes.
It's a really good series, and I really enjoyed watching it. Even though it is really for young kids, I still enjoy watching it (for some reason). It's a great show. Check it out!