Dragon Quest IX is a good game and I've been playing it nearly every day since I bought it three months ago (an hour each day or so). The fact that it has sold more units than others must stem from the fact that it is for a portable platform however, because compared to previous installments it is pretty much a step back in gameplay value. For one it has a serious variation problem during the main game. You're always doing the same and you don't have any quest that crosses town boundaries, except for the occasional single shallow dungeon (a problem of more RPGs nowadays, just look at Fallout 3 for example).

The real deal with this game starts when you finish the main quest and you get into boss grinding and doing power leveling. If you like to invest lots of time into reaching goals you set for yourself (get all achievements, complete list X, beat all legacy bosses, etc.), then this game is for you. If you want an RPG with some decent multiplayer options, this is the game for you. If you want it for the single player main game content, it is currently too expensive for what you get.