Back when FFI first came out, it was awesome for it's time. Now a days, I would only recommend if you are a serious Final Fantasy fan or a serious RPG player and do not care about story or character development but difficulty. The story is not bad, but now a days it is very out dated. And the characters have no personality of their own.
FFI is not a bad game, but people who grew up with the later Final Fantasy games will probably not enjoy it. It may be worth checking out to see where the series started, but I don't know about buying it.
Only recommended for serious RPG fans. Or maybe if you really want a GBA game to play in your spare time. Just have a walk through if you get frusterated very easily.
If you really want to try it out, I would suggest downloading it to an emulator and trying it out.