While it is a huge step up from Super Mario 3D Land, it still uses the same general race-to-the-flagpole system rather than the more exploration-based star/shine system in the 3D Mario games before 3D Land. But the multiplayer is quite fun, although I didn't experience much of it due to not having many friends who play Mario games.
I never made it to Champion's Road, being stuck on the second green star of Piranha Creeper Creek After Dark, among a few others. But I can say from videos of it that it looks like hell. I never beat The Perfect Run in Super Mario Galaxy 2, so I doubt I would be able to beat Champion's Road.
The game has nice music, and the Galaxy remixes are amazing. The bosses, for the most part, are more interesting than those in 3D Land, and the final battle, while sadly still "run-from-Bowser-and-hit-a-switch/block," has amazing music and is very climatic regardless.
As for the Captain Toad levels, they are not my cup of tea. I can beat most of them, but some of the later ones are hell. The last one in World Crown looks impossible for me. But they are by no means bad, and I appreciate the bonus levels regardless. Overall I think Super Mario 3D World is a solid game, I prefer Mario 64 and the Galaxy games, but that's probably mostly from nostalgia. I rate it 8/10, amazing.
^Not much of a good review, but its what I could come up with in a short time period. Hope it helps.