I'm a bit of a fan of the Sims series, and had almost completed the GBA version when my DS version arrived. I expected the graphics to be more in line with the console versions of the game, and was quite disappointed to find that 90% of the graphics were identical to the GBA version, despite the DS being capable of better. The faces of the characters have been slightly improved, but graphically everything else is the same. The sound is also the same as the GBA version. I think the game could have benefitted by taking advantage of the DS's 3D capabilities.
The game does take advantage of the touch screen in some ways, as it is used for the menus, which contain your characters stats, items, the map, options etc, and is also used to control some of the mini-games. It can be annoying to switch between using the buttons and the stylus, but I got used to it eventually. The DS version also has a new area and several new mini-games as well as “virtual pets” which didn't feature in the GBA version. It takes a while to get to the “new” bits however.
My favourite thing about the handheld versions is the RPG-ish feel that the game has compared to other versions. The game works on a mission based system, once a series of missions is completed, a new area is usually unlocked. In addition to this you need to keep your Urb healthy and happy, and earn money to survive, so this game should keep you occupied for a while.
Despite its relative lack of improvement over the GBA version, I think the Urbz is still worth owning, overall it just seems like it was a bit rushed.