Matthew and Ross, both 13, from Bedfordshire, were down at the front of the stage.
"The music was brilliant," Matthew told Newsround Online, "but there was lots of pushing and kicking in the mosh pit. We nearly fell over!"
"I really like the masks," said Ross, "the band could walk out in the crowd right now without the masks on and no-one would know it was them."
In some ways, Slipknot are more interesting to watch than to listen to.
The nine band members have amazing energy and they create such a huge wall of noise.