I'll admit it, I enjoy watching Geass.
It's drama was neither over-the-top nor boring.
Through ignorance of a 'weak plot', one can sit back and enjoy the actions of the many characters within Code Geass that incite emotional connection to it's viewers (either good or bad).
Comparing it to Gurren Lagann is very difficult. They are both created for enjoyment, yet Code Geass most definitely does not try to enforce imagery or make the story 'DEEP' or highly worth critical acclaim (i.e. previous point about plot).
Why do people dislike Code Geass? Due to their personal tastes disagreeing with the incoherent plot; most likely.
Why do people like Geass? Reasonably good animation/character designs, a story we want to see the resolve of, and the several characters who the director/writings have created to entice YOU the viewer.
It's sheer brilliance; and you should stop bitching about Code Geass the anime, and only "/a/ - Code Geass".