Honestly, I consider the Lunar Trials to be the weakest of the new optional content from the GBA titles. While I can't speak for the ones in FFI-III, I can say the ones in V and VI are excellent. V has you go into an abyss world that is basically the final dungeon on steroids with the final confrontation being against Enuo, the warlock that summoned the Void in the first place. There is even a floor where you can fight multiple Omega Weapons before tackling a souped up version of it.

VI has the Dragon's Den, which is a multi-party three team dungeon where the party can go head to head with very well thought out super versions of the Eight Dragons. These fights are hard and make having spells like Quick, ReRaise, and Ultima absolutely necessary for victory. You get to fight the Kaiser Dragon at the end, who was a super boss that can be found in the original game's code, but was never finished. In fact the DD seems to be the team finally finishing some of the ideas for the Eight Dragons side quest because the code detailed there was suppose to be more to them.