Mystery Dungeon is an awesome game. However, I only gave it a 7/10. Don't let this discourage you. I just happen to enjoy competitive battling a little better, and once your Pokemon get strong (like mine), you can kill anything with a normal attack which takes away from the strategy of the game.
Yes, after you beat it completely, there isn't really much to do, but that is what the "I died, I need help" thread is for. As long as there are rescues coming, you will never get bored with this game. I haven't completed it yet, but I have completed other games before, and I know what it will be like when I complete this one. However, what's cool about this game as far as completion goes is that it takes an extremely long time to complete, so that stage is still far off.
I would recommend buying it. It is the only game where you can be your favorite pokemon, and it won't get boring as long as there are rescues needed. Even though a Mystery Dungeon 2 is coming out, I would still buy this one because the story of Mystery Dungeon 2 may be based on the first one. Also, the story mode alone of Mystery Dungeon makes it worth buying. It's the only Pokemon game with an awesome storyline.