Levels 2-4 felt a bit boring. 5 and on were more interesting though. I was having more fun when I didn't have an easy spot to camp out. I only played once though and got through to level 8 with only 4 enemies left.
Reading the instructions again, I get what the powerup description is saying, but it wasn't clear enough for me to understand when I was playing. I usually couldn't find a safe use for it anyway since there's no indication of when it ends.