I haven't ever come close to completing the entire sphere grid, although my brother has come quite a ways, he has activated every node with three of the characters (Tidus, Yuna, and Kimahri), and he's changed tons of the nodes using spheres (example: clearing a HP +200 node, and replacing it with an HP +300 using an HP Sphere, and doing the same for the other stats, including Luck).

Took him quite a bit of time to do just that alone, but he has three of the characters having over 96000 HP, so it's proved worth it, he can beat Nemesis consistently.