Yes, it's well worth it. If you already have the SNES version, here are some advantages to buying it:
  • Portability
  • Much, much better translation
  • Many previously excluded items are back
  • Ability to choose your party at the final stretch of the game
  • Post end-game extras specific to FFIV Advance
It's easily the best RPG available on the GBA at this point. Also it should be noted that in the near future Final Fantasy V and VI will all be available on the GBA and it's going to be nice to have the whole set.