You know I feel the same way. Around the last two or three levels it dragged. I'm just starting the golden puzzles now, and it's definitely falling off. But those first, say, 10 levels were great.
I did multiple playthroughs, over ~150h. I do not expect to return to it, now that EDF6 is out. It reaaaaaally resembles EDF5. From what I have seen thus far, it could be called an "expandalone". There is much more difference between EDF4.1 and EDF5 than EDF5 and EDF6