If you two are casual gamers.....I wouldn't look into getting into this game. If checking it out involves putting 2-3 years into the game to feel like you've "beat" it then go for it but like the others said, this game has a steep learning curve and is all about grinding.
There's some great aspects of this game that I really enjoyed and would like to see in other games, like multi-class and sub-class on a single character. This eliminates the need for any alts and repetitive gear farming. The combat system was also top-notch and complex, although a bit rigid compared to current games like WoW and LOTRO. Anyways, combat and the dynamics involved in this game still make other games look like novice easy-mode.
Graphics as others said are a bit dated, but still look excellent. FFXI scales incredibly well with higher resolutions, you just need to hack the .ini and change the hex values for your 3D background resolution. I played at 1920x1200 on my 8800GTS at the time I quit and the game still looked marvelous (its only DX8). Many who left for or tried WoW said FFXI still looks far superior. LOTRO's graphic engine is certainly much better but FFXI's art direction makes it the better looking game in many regards.
Some of the darker sides of the game involve a very heavy reliance on grouping. Everything from EXP (grinding EXP can be considered a large game aspect), to grouping to kill HNMs to gain loot, to mass grouping to get a chance to drop rare/artifact armor/gear. Not only is there a heavy reliance on grouping and competing for World spawns, the spawns are not instanced and spawn randomly at all times of day. Oh, did I mention this game is heavily international with a massive Japanese population? Let's just say the game mechanics lend themselves to some pretty hectic playtimes.......
Anyways, if your gf enjoys the console FF games, I'd recommend sticking to those. You can pick those up and put them down in 30-40 hours and play through them again at your leisure. FFXI isn't like them at all.....and although I've heard they've made some changes I'm not sure you want to get started on that game. Keep in mind this is probably my favorite gaming experience ever, I figured I was doing you a favor.