mario rpg, paper mario 64 and super paper mario are all quite different. I loved mario rpg, found paper mario 64 mediocre and really enjoyed super paper mario.
The levels of super paper mario are great, and the flip mechanics are solid- wish there was even more to it. If you have played the psp game crush, it is a very refreshing way of thinking about how to play videogames.
The hub world and between levels is rather boring. There is stuff to do and explore but I passed most of it over since I wanted to play the next levels. The story is one of the few mario stories I remember and some of the dialogue, whether intentional or not, is hilariously awful.
It is a good deal of fun to play, if I hadn't bought it at $50, I would definitely get it now at $40.