I don't really agree with Blue Spheres as a mid-level bonus stage. The 3 bonus stages in Sonic 3&K at most lasted 1 minute, but often averaged to around 30 seconds, it was a quick break and didn't ruin the flow when you stopped to jump in one. Blue Sphere levels are all pretty lengthy, and a 2 minute (or more!) pit stop mid-level can really cause some burnout while playing.