I got it, and it is totally worth it! I must say, it is a little challenging. I just finished the Mid-Fortress in World 2.
There are many things to do and unlock (a trapped toad pops up once you beat the fortress. You have to carry him through the level, which is a challenge. Doing this gives you a "rare item." I've done it once and gotten a star that I can enter a level with).
The only thing that I find interesting is that if you pass the check-point as normal Mario, it has the same effect as getting a Mushroom.
I think I may start a review of the game before I finish it. It's just too awesome to forget any of the points I've made so far... especially with the World 4 Castle. Three words, GINORMO SPIKE PILLAR.