Completed the 4th Key Temple (Jagged Jewels) and the Cave world in general... haven't played 4-B yet, though. I really liked the mine cart stages, which I couldn't stand in the SNES games, but the rocket barrel levels just kinda suck, mainly because of the controls that demand you be accurate and quick... neither of which the rocket barrel is. You end up having to rely more on memorization than you do actual skill.
The silver lining here is that I feel pretty good when replaying the stage to pick up stray puzzle pieces later and fly through them without dying... then I think, "How did I have so much trouble with this again?" I still lost around 40-50 lives over both of those rocket barrel stages.
And these get harder? Oh man...