3DS is backward compatible with the DS games. In fact, the 3DS has 2 different resolution set-ups for your DS games, one being stretched due to the 3DS' larger top screen and other being normal like on your DS/DSi/DSi XL systems. 
I got Super Street Fighter and man... it's hard to describe on how 3D works, but it's like a watching through a window and sometimes has a "pop out" effect. Let's look at it this way, the 3D has different depth levels from looking at a far away tree/flower/car/etc and having a person right in front of you.
Got Bowser's Inside story and it's a great game that requires you to use your stylus at times, timing/strategic skills, and has a funny storyline. :P
