Powerful! A direct hit on bare skin will make you yelp. This is a GOOD thing. Glad I also bought a box of safety glasses.

UPDATE at 8 months: We've taken the insert out now and use it in our stroller (Bumbleride Indie, fits perfectly). Our baby still enjoys her time in the Mamaroo and now its a great "mini-Lazyboy" for her to watch a bit of David Attenborough while I fold clothes or cook. Still falls asleep in it but we just move her to her crib. Our Fisher Price swing hasn't been used in months (she disliked the position, plus the sounds broke) but the Mamaroo is still going strong. She kicks in it A LOT, sometimes we think she'll break it, but she never has. We moved recently and it packed easily right back into its box, the size of a laptop box.