I've been playing this! Well I was, until I was a total dope and accidentally erased my save file (long story). Luckily, Tomato himself had a save fairly close to where I was and he gave it to me. I just need to pick it back up again.
This is definitely worth playing IMO. It is much more difficult, but to compensate many bosses and enemies have status and elemental weaknesses they didn't have before. One optional boss in particular was slaughtering me until I figured out they were vulnerable to Stop, which basically turns the battle into a cakewalk. Also, stat growth isn't dependent on espers anymore - you just level up your stats like you do in most other RPGs -so grinding is a viable option now.
The text is based on the GBA port where it can. Aside from the new content, there are a few lines sprinkled in here and there which helps flesh out the characters a bit. For example, Cyan has new dialogue if he's in your party at times that you confront Kefka. All the additions feel very natural without going into any tonal dissonance or excessive fanservice.
There's loads and loads of new music, most taken from other RPGs. Some of it may sound kind of funny depending on your emulator but I'll be honest I got pretty hyped hearing Meridian Dance during one particular boss fight.
If you're a FFVI fan I can't recommend this enough. My only complaint is that while I don't mind grinding levels, some of the bonus content is hidden behind achievements which are incredibly grindy. For example, one bonus requires you to use Cele's Runic command successfully 500 times - which takes forever and something you'd never hit unless you were purposely grinding for it.