It plays almost exactly like the GBA one, except now there's a status screen at the top. You can get through it, but it would really help to be able to read Japanese. Also, the graphics are a bit crisper than the GBA one, but that's probably just because of the screen.
You walk around on different levels of the dungeon and fight with other groups of wandering pokemon. There's no trainers or anything so far. So, yeah.