The best free MMO I've ever played!
Almost any monster (Demon) you run across on the playing field can be tamed and used in battle, and all demons are directly controllable and have huge varieties of skills and abilities. The demons you tame can also be fused together to create completely new demons, and the various demon types can be collected in a sort of encyclopedia in a similar style to Pokemon. There is also a main story to follow with plenty of quests for those who don't like their mmorpgs to be never ending and lacking in direction. There's also a dungeon area where instances can be randomly generated and private groups can do some demon slaying/item collecting. Crafting exists, but at this point I have not had the time to get into it other than to collect lots of the materials needed for it from "Plasma" orbs, which start a quick timing minigame once clicked, taking a little bit of the boredom off of the usual node farming seen in other mmorpgs.
Your avatar's skills are improved simply by using them and there are no classes, allowing complete freedom of character development.
Battle is an entertaining affair involving some rock/paper/scissors style weaknesses, attacks and blocks and is something that is easy to learn, yet difficult to master. This game removes itself from others in the series by throwing out the turn based gameplay and inventing something entirely new that flows much faster and allows other players to assist with great effect.
One of the few drawbacks to the gameplay, however, is that the active battle system can be a bit clunky when attempting to control both your demon and your avatar.
The game is overall very similar to the ever popular Pokemon, but this isn't a kid's game and does not have the cutesy style of graphics as seen in Pokemon. For example, one of the demons, Angel, is almost completely naked and wearing nothing but a bondage harness. The atmosphere is also very dark and the game takes place in a post-apocalyptic world to further reflect the unique feel. The graphical quality is on par with the other PS2 games in the series. It shows its age, but not enough to complain, especially when compared to other free games out there.
At this time I have not yet finished the main story as I've been completely immersed in the demon collecting and dungeon crawling, so I cannot say how long it is, nor reflect on the quality of later areas.
Overall, I would give the game a 9 for being a cut above the rest.