Just beat Rainbow Curse today.
My thoughts on it; The first few worlds are really, really slow, but once the pace picks up, the game improves a bunch. Appearing to be an even slower Canvas Curse is not a good first impression, but if that's the impression you got and put the game down, I think you should give it a little more time.
The last few worlds were challenging enough (collecting everything) to be really entertaining. I even found a use for Kirby Amiibo's unlimited stardash in a later stage. The visuals and soundtrack are where the game really stand out. I didn't play on the television much, but other people watching me did really enjoy it. And I didn't feel like playing on the pad took away from my experience.
Challenge mode is really lovely too and if you thought the game was way too easy, you may find some enjoyment here. They take what you learned while playing the game and cranks it up, giving you a series of rooms you need to complete in a short amount of time. This is not as visually appealing as the story mode, but it's really what I love. I didn't have the chance to try multiplayer in either mode, which is a bit odd because I've been playing through all the Mario titles recently with 2-3 players.
I think it's easy to compare this game to Captain Toad. They're both very nice looking games, although I think Rainbow Curse stands out far more. Rainbow Curse also has a much better soundtrack and general variety. Both games have Challenge Modes and Captain Toad is a bit more varied, but they're both very fun in their own way.
Overall, I think it's a good game that won't appeal to everyone, but you'll probably know if it'll be for you, or not before you ever touch it. I am probably much more forgiving on some of the game's faults because of the price point. Nintendo creating a 'budget line' with games like Captain Toad and Rainbow Curse seems really smart for me. Where I might pass on one of those titles for $50, or $60, it's almost a no brainer for me at $40.