I got this game yesterday for the nintendo DS. Actually, it's been getting some pretty spectacular reviews across all the major video game sites. And I too have been mightily impressed by what they were able to do for the DS.
It's not some cheesy half assed imitation of the GTA experience. There are some obvious differences, namely the inferior graphics engine of the DS means that obviously they couldn't make it look like the Playstation versions (or even the PSP). However, they made some clever 'modifications' and adaptations, and used a cel shaded type of animation, along with a hybrid top down view. The classic GTA type of missions are there, drug dealing, hits, trying to make it in the city. The music and sound effects are excellent. I've read that this is almost 15-20 hours of gameplay. There's no denying that even though it looks different graphically, you still feel like you're playing a GTA game. Highly recommended.