It's fun but it's one-track-mind fun with little depth.
Once the cuteness fades and you become accustomed to the atmosphere, what's left is of course the game-play. Loco Roco has you trying to find and guide as many of the world's inhabitants to the end each level as possible. On your way, you'll have to traverse mildly challenging landscapes, and a set of adversaries that prove more of a nuisance than a challenge. The game plays out as a standard 2D platformer, where you can move left, move right, and jump. The environment can hurt or help your motility with a variety surfaces that are smooth, bumpy, spiky, slippery, etc.
Over the 5 worlds (40 levels) there's not a lot to differentiate them. Levels start getting predictable early on with not a lot to keep things impressive and engaging. A lot of the charm and intrigue wears off before you're half way through. There is a kinda-sorta-boss-level that ends off the game, but more instances of something like that would have been appreciated.
If your main goal is just to beat the game then you should be able to do so without much stress in about 5 hours if not less. The biggest challenge would come to those looking to collect all the items there are available to collect and get the best score possible. Completionist should be kept busy for a while longer, although myself I'm not dying to go back and replay the levels.
With your collectables and unlockables you've achieve you can work on modeling a one-screen fun park for an NPC Loco to go roaming around, and potentially collect even more items for the Loco House. They did add some bonuses like unlockable mini-games which allow you to collect even more items, but I found all of these a little forgettable.
It could have done with some more puzzles and some more variety, but it was entertaining enough to have a little innocent fun and kill some time.