The real issue here
I remember seeing the Dark Phoenix saga on the X-men cartoons, and read about Jean Grey's ongoing problem with her super-powerful side from wikipedia. The real issue with the Dark Phoenix saga is the fact that basically, Jean Grey turns into an almost unstoppable, dangerous entity that can't truly be killed, and she hogs the spotlight, drowning out all the other characters as they struggle to deal with her.
I've never enjoyed that particular version of Jean Grey. She eventually falls into the "too dangerous to live" category, and makes fans and friends mourn because she was a lovable, strong character and a force for good. But once her phoenix side comes out, she becomes something else entirely.
It is partially Professor X's fault for not allowing her to learn to harness this side of herself early on (at least, according to the movies).