Five worlds in, my verdict on Sticker Star is that it feels incomplete. The hub is populated by identical, nameless Toads who have little of interest to say. This, in tandem with the infrequency of Kersti's interjections and the omission of partners, adds up to a feeling of emptiness by Mario RPG standards. Even the more platforming-oriented Super Paper Mario features more character interaction.
I don't understand Nintendo's recent insistence on dividing Mario locations into linear stages, as also seen in SPM and SM3DL. The gaming community let out a collective groan upon seeing "level complete" fanfare in an early trailer. Maybe it leaves less room for design error or disguises the fact that the game doesn't really have any characters. Whatever the case, it's a cop-out, and nobody appreciates it.
All that said, the basics of a decent game are all present. Just don't expect it to be another TTYD.