Well, to its credit, it's not all rehash. the final battle of the movie is done completely different.
Remeraber when Simon gets over his depression with that "My bro is dead. But I'm me, Simon the digger!" speech? That whole battle has Team Dai-Gurren taking on Viral, Adiane, Cytomander, and Guame at the same time. Some things happen differently too, such as Simon clirabing up one of the generals' mechs with his core drill in an attempt to save Nia. Yoko also personally gets into a fight against Adiane.
That said, I'll agree that it glosses over too much. On the bright side, movie 2 is probably going to turn out being more different than the first one. By killing all the generals already, they've already made changes to how the big battle against the spiral king will go.