It's a great 3D Mario game, probably the best since Galaxy 1 and 2. I felt 3D world lacked a bit in some innovation and was more or less just "more 3D Mario with nothing special". So far through most the kingdoms doing a fair amount of side stuff and such. Some kingdoms are better than others, some seem to be more repeats of similar themes, some very unique ones, some fan service type ones. Overall the cap mechanic is really neat I like it.
If I had to pick anything with this game, it's maybe not as challenging as Galaxy was, and the music is not the best. In particularly, it seems the music actually starts playing and getting good at the end of most kingdoms...after you beat the boss and done most things, which is kind of a bummer.