The Monster Arena is good in principle... but it gets so convoluted and super super endgame that it basically turns into a mess. Seriously, you pretty much need maxed sphere grid, with most if not all the empty nodes coloured in to have any hope of doing well in the later creations/Dark Aeons/Penance. You might cope with the species creations at a lower grid though: I beat Chimerageist my first time in the Calm Lands actually. Really tricky to beat him during the first visit though, but I did it.

Pretty much every other creation requires further, advanced levelling. The Magus Sisters can act as a crutch of sorts for awhile, but eventually you won't need them. After that, it's just mindless farming for spheres. It gets dull, incredibly early. The music is dull, combat is dull, everything is dull.

FFX's main sidequest turns the battle system, which was fine and dandy, into abit alot of a chore.