Finished on co-op with friends. I haven't had this much with 2d Mario since the 'new' formula started on ds. There were some levels that felt too easy and straight up filler. Others that were a good challenge. And others that only felt more difficult because of co-op mode. Despite this mixed bag, I was consistently amused by the gimmicks and the stylistic choices introduced by the wonder seeds.
They enhance the game's overall design as it keeps most levels from feeling static or samey like some of the latter 'New Super' entries. Not to mention some levels have some really well hidden wonder seeds you will probably miss on your first go, offering some decent replayability value.
Overall a very solid and snappy return to form for 2d Mario. Opinion will probably improve once I play through It solo.