I've been a big fan of the sanbox genre since GTAIII which is my favourite game of all time.
Chinatown works really well on the DS, utillizing the consoles features while keeping the Grand Theft Auto magic.
Everything is there, and more. They've brought bask the old taxi missions which were wrongfully left out of GTAIV. The game has been seriously updated to include things like a PDA and home delivery from AmmuNation. All these little touches set the game apart from it's 3D counterparts and gives it it's own identity.
The touch screen has been implimented really well too. Everything is a mini-game like making Molotovs to searching dust bins for weapons.
I didn't find the game that hard but it's fun and there's a lot to do. If like me you like to spend a lot of time mucking about in a sandbox game then this should last you ages.
The only problem I had was with the cut scenes. You've got to read all the dialogue, however that's the same with almost all DS games. Sound wise it's pretty damn good from the sound effects when you DS console from sleep to the pretty decent radio channels.
Buy it, you've got to. Argueably the best game on the DS.