I've been playing this on and off, for about 10 months.
The first month was awesome, because it's fucking old-school megaten in 3D (the soundtrack even has some great remixes from SMT2). In fact, the story takes place between SMT1 and 2, and tries to explain what happened during the 50 years or so separating the two games (the rise of Arcadia, etc...).
But the grind becomes intolerable after level 40. The game only has 6 dungeons, only 2 or 3 of them are viable grinding spots (and you still have to run them 6-7 times just to gain one level). The world is also very, very small (there are a lot of unopened areas, though, but it's still kinda disappointing for a game that was released 18 months ago).
It's a shame, because there's a cool crafting/item customization system, and fusing demons is awesome. Also the battle system is fun (when you have a full party struggling against 30+ demons spamming AoE and shit).
If you want to give it a try before the US/euro beta, most english-speaking players are on the Kerberos server. We even have a few guilds .
XI Wiki


