I finished this recently (Japanese version), my total play time was about 20 hours.
Initially I found the game fun and charming. As the game progressed, I enjoyed it less and less, though it was always "good enough" for me to keep going and finish it. At its core the game is very much an adventure game in the vein of something like Secret of Monkey Island, but they toss in very weak RPG elements and very weak platforming elements. It's just not a great end product, and I found the sticker mechanic underwhelming and very repetitive.
The story is simplistic and unimaginative: just your typical derivative Mario stuff that's been shoved down our throats countless times. I expected more from an adventure game. The funny thing is, many times while playing this I kept thinking to myself "Man, this game would be so much better if it were an original IP not in the Mario universe, with a more fleshed out story." I guess the Mario overload on the platform is really starting to get to me.
A lot of people heap praises on the music. I found it to be good overall, but it never wowed me.
Overall, I'd give it a 6 or 7 out of ten (so let's say a 6.5). Just barely decent enough to push you along to the end, but far from a great game (I will definitely never replay it). Some stages and scenes were genuinely enjoyable and creative, but I found them few and far between.