I'll spare you the nuts-to-soup FF6 review, as it probably is completely unnecessary. Personally, I think it's the peak of the Final Fantasy series, with pretty much everything going downhill from here (though FFX was pretty okay, and admittedly I haven't even played FFXII yet). For its time the graphics and music were astounding, yet it didn't come at the expense of gameplay (FF6 is smoother than any FF game that came before it). It also had exceptional depth, with absolute oodles of hidden side quests and optional things that didn't involve intense level grindery. The game's tone struck a nice balance between lighthearted and serious, in contrast to the overly angsty efforts we would see next (and then the overly saccharine FF9 in a misguided attempt to bring balance back to the Force or whatever). And it busted out a lot of sequences that will seem tame now, but were pretty mindblowing back at the time of it's release. Most importantly, it's aged really well - still looks nice, still sounds nice, still fun to play (if not as overwhelmingly awesome as it used to seem).
What I just want to look at here is the differences between this and the SNES version and the "new content", and hit upon some points I didn't really see the major game magazines address adequately when this came out about a year ago.
First of all, as compared to the SNES version, it's pretty similar. Nothing major has been excised. The script has been re-translated and some missing dialogue from the Japanese version restored, and the Esper names have been restored to what they were in the original release (player names haven't been Japanized though - no Tina and Mash, though I guess you can rename them yourself if you're insane like that). Oddly, some things have been uncensored, but a new bit of censorship has been introduced. Square and Nintendo seem to have just skewed from censoring sex, to censoring violence. Now, FF6 wasn't exactly a ribald game in it's original form, but there are some very tame scenes where a hooker offers her breasts to Cyan and Relm makes some innuendos that have been restored in this version. However, the scene where Celes is repeatedly being Barlog punched in the stomach by an Imperial guard has been edited to have the punching animation removed, and Shadow is no longer willing to slit his mama's throat for a nickel. Neither of these scenes really impacts the game all that much, though some people raised a ruckus on the interweb about them when the game first came out. Overall the new script is pretty decent. It definitely seems more verbose than the original, though I don't know if it's actually better. Some of Woolsley's famous lines have been left in, but other good ones have been dropped. I saw some people on GameFAQs going a little nuts about how Locke is too forward with Celes now, I can't say I really even noticed the difference personally, I guess I never got into the story to that level. So basically, unless you're some kind of incredibly demanding purist, the script and dialogue probably won't disappoint you.
The graphics have been improved much less in this game than they have in the other ported FF titles, but then they were also in much less need of it since this game looks leaps and bounds better than any of the games that preceded it. Unfortunately, this game was very heavy on Mode 7 effects, which is something the GBA is not capable of directly emulating. They've worked around it by using basic scaling, like what Sega used to do in their first-person games such as Space Harrier and Afterburner. These sequences have definitely taken a noticeable hit. Amazingly, the airship scenes aren't affected too much, though you'll notice some "cracking" in the sky backgrounds when you make a turn. Most of the spell effects that use Mode 7 look pretty similar as well, just sometimes not quite as smooth (like they've had frames chopped out of the animations). Unfortunately, there are certain effects and sequences that look very noticeably uglier and more limited. The most standout examples are the Serpent Trench, the mine cart ride through Vector (horrendously ugly), and the ending credits sequence. These things are disappointing, but not enough to not recommend the game.
The music has taken a much more noticeable hit than the graphics. The SNES had a pretty amazing sound chip and the GBA ... well, it doesn't. They did a pretty good job making the game sound like the original, but some things just really couldn't be helped I guess, due to the limitations of the system. If you're familiar with the original soundtrack, you'll notice a number of sound samples that are lower quality. The game particularly struggles with songs that were heavy on bass, loud percussion and "unusual" effects - so this means that most of the battle themes, the opera, the final dungeon and the second airship theme are among the songs that have taken the most noticeable dives in quality. Songs that are lighter and use strings/woodwinds primarily, such as the town themes, sound virtually identical however. On the whole, it's really not that bad, but SNES vets will notice the drop in quality.
Other small tics - obviously, the graphic and sound deterioration has caused the game's epic ending to lose a little of it's power. What's even worse, though, is that the music goes out of sync with the ending for the last minute or so, leaving the very last few scenes in absolute silence. There's also a touch of slowdown here and there throughout the game (particularly when Mode 7 effects are being emulated, and when big battle sprites or very busy backgrounds are present).
After getting all the negatives out of the way, we finally get to the primary draw of this release - the New Content. Aside from the requisite bestiary and music box, the game has added four new Espers to the mix and two new dungeons. The new Espers are all optional and fairly powerful, found towards the end of the game. A couple of them add new spells to the game, though none of the new spells can out-cheese the classic Ultima. Mostly, they're only good for providing better stat boosts than most of the original Espers. The two new dungeons are accesible after you kill the eight dragons (you may have to also finish the game once, I'm not certain on that one). The first dungeon is the Dragon's Den, which is a three-party affair similar to the Phoenix Cave and the final dungeon. As far as the design goes, it isn't particularly inspiring - mostly just a series of battles against cranked-up versions of the eight dragons which require you to have the right elemental armors equipped beforehand. At the end you take on the Kaiser Dragon, a new uber-enemy who supposedly is more powerful than the final boss. Beating him gives you the game's final hidden magicite, Diabolos, and opens up the second hidden dungeon - the Soul Shrine, which is actually not even a dungeon but a gauntlet of 128 battles against tough foes with only a few breaks to rest. So essentialy, the "new content" in this one is just a whole lotta fighting against mostly palette-swapped foes. No new characters, further character development or even neat designs like FF4A's Lunar Ruins.
So, is this the "definitive" version of the game? I think the answer is definitely No. The SNES original is still the way to go. This one's rather piddly amount of "new content" doesn't mitigate the blows it takes in terms of graphics and sound, and the game itself has hardly been renovated at all. It does work well in one way though - as portable FF6. The conversion is good enough to make it worthwhile for that purpose. If you want to sit down and play it at home though, the SNES (or at least, the emulation of the SNES) is still the version of choice.