It may looks silly on the begining, but Digimon World DS gets highly deep and addictive as it goes on...
The farm system is very cool, and customizing it to get the necessary types of EXP, and the highest grow rate of your Digimon adds some diversity to the game! The various Evolution Routes can take the Replay high to the skies, and the various combinations of itens and abilities, the types of Digimon, the Tamer Rank... All this things make Digimon DS the first deep monster-breed game to come.
The bad part is that the graphics are very basic, and the game could have made much more use of the NDS capacities.
But, for those who say that Digimon DS is just a game to wait for Pokémon, it looks like Pokémon P/D won´t make good use of the DS graphic capacities too.
Digimon DS ends up being a very immersive game, with criative raising mechanics and a very open monster-customize system!