Ethers and Stat boost potions tend to be fine and usable because their effects have not diminished as ilevels have increased unlike what happens with HP potions. An X-Ether still allows you to cast a spell the same number of additional times whether you are i55 or i130. An X-Potion on the other hand has diminished from 24% of a character's HP at i50 to less than 10% at i130.