It's a classic. I don't know how else to summarize it.
The gameplay can be anywhere from simple to quite involved. Enemies come in huge quantity, and simply avoiding getting completely overwhelmed can be a challenge sometimes. It's a very loose game in that you can take it at your own pace, and go off and do your own thing instead of following the main storyline. Characters aren't limited much in their development (unlike future MM games) beyond what they can initially learn and their base hp/mp, allowing for much flexibility. Even after completing the game one can roam around as they see fit, and there is a veritable ton of easter eggs to be found around the world.
The interface cannot be customized, which is a drag, but it is not too hard to get used to things as they are.
There are some balance issues. Melee classes dominate in the beginning and magic classes take prominence toward the end... but every class has its uses throughout, and doing well requires many specific roles to be filled, in both skills and spells. Some spells are decent in the beginning but do not scale well at all in power, a problem later titles fix. However, your casters will always have certain spells they can always fall back on. In short, you can deal with them.
The difficulty factor is hard to put a finger on. You'll often be cruising fine once you get the hang of the game, but if you lose a key member of your party, things often can go south in a hurry. Water mastery makes the game a lot simpler both by easing travel and allowing for quick escapes and quicker returns through placing beacons, but that is something most aren't able to do until at least midgame. Status effects severely complicate things at times, and monsters that can completely drain your mana, instantly kill, bring one to 0 hp, or eradicate at random can prove difficult no matter how tough your characters are.
The graphics are okay. 2D sprites on 3D backgrounds naturally look somewhat awkward, and some spell animations are reused often. Texture shading as the sole means (well, save size) to indicate power variations among different variants in a family of creatures is also kind of tacky in my opinion. Nevertheless, the art is still very well done, despite the shortcomings of its implementation.
The sound effects are very lively and have quite a lot of character. The party's voice acting can be a bit over the top at times; I think this only adds to the flavor of the game, however. The music gives a sort of storybook feel, and it is only used limitedly, meaning it establishes atmosphere without growing irritating.
All in all, this game provides hours on end of quality gameplay, if you give it a chance. If Ubisoft ever does anything with the rights to this franchise, they should look to this game for inspiration.