Sim City Jr. is a nice little diversion from the Sim series.
It's not quite as hard as any of the other games, but does provide some nice quirks. The fact that the game is easier and you don't have to manage your resources nearly as tight means that you can mess around allot more with the editor itself (something I used to do quite often). I'd make pictures with the trees, or create rather elaborate paths.
While not everything is done right, the game does reward you quite well. When you play, you rarely see cars; however, when you do, you get a rather accomplished feeling inside because it means that your town is really REALLY thriving. (granted with those kinds of features, it should be called SimTown Jr.)
So all in all, the game is worth playing if you see it on your friend's desk or old CD collection, but don't go out of your way to buy it. I'd certainly recommend SimCity for SNES though :D