Alright, I've been reading a lot of the comments, including Ian's detailed rants, and this is my opinion on the overall franchise...
As I already said in the SMRPG entry, that game is a masterpiece of gameplay design, visuals, music and even story. When Square and Nintendo finished the game they broke the mold they use to create this title, making sure that no other game would be quite like it ever again. They shot themselves in the foot with this, however, as now any Mario RPG that is released is directly compared to SMRPG rather than just judging the games based on their own merits. In other words, SMRPG was such an amazing masterpiece that people can't help but expect any Mario RPG to look, sound and play EXACTLY like it. I admit I was under the same spell when the first Paper Mario was released. SMRPG played such a big role in my enjoyment of video games that I couldn't help but see Paper Mario as a very weak follow up. Up until recently, however, I realized that trying to expect an experience that will be exactly like the one I enjoyed in my childhood days is very foolish and should just have fun and enjoy the ride.
Now I am able to appreciate the series as a whole. To me, the Paper Mario series is indeed a very simple RPG designed for younger gamers and non RPG fans in mind, but I really like the aesthetic of a world made entirely out of paper, and on the whole, it is adorable. I love the Mario and Luigi series because it gives the characters a lot of personality and sometimes that is enough to make the game really enjoyable. I mean, people have called "Partners in Time" the weakest in the series, but even then I had a ton of fun with that.
Regarding Super Paper Mario, I liked that game, but gameplay wise the overall designed is flawed. The idea of 2D and 3D gameplay is ingenious, but the developers didn't know how to create a more streamlined menu system, meaning that constantly changing characters, switching between planes and changing fairy characters was a chore, and one you did often. The main reason I kept up with it was because the story was really, really good. Like I mentioned sometimes a story and character is enough to keep pushing forward. I really loved how the story poked fun at itself as well as other Nintendo conventions, and was considerably more edgier than past Paper Mario titles. I like it just for that alone.