I also have the Keyfit 30. We took ours out after LO had a huge blowout in his car seat. I looked in the manual to see how to wash it and realized LO had outgrown it!
We just took ours out, also due to a blow out. We watched him carefully on the ride home and he was fine. Head was fully stable so we won't put it back in.
We took the insert out at about 11 pounds (KeyFit 30), and he seems SO much more comfortable without it. He doesn't scream as much when we strap him in now. We still have the head stabilizer in though, because he doesn't seem to mind that, and still has trouble holding his head straight when he's tired.
Re: Infant car seat insert
Natural miscarriage @ 8 weeks - 3/8/2005
Big Brother "Skippy" born - 2/28/2007
Missed miscarriage - (EDD 3/5/2013) - D&E @ 11 weeks - 8/8/2012
"Hen" (EDD 6/7/2013) - born sleeping @ 19 weeks - 1/15/2013
"G-Unit" born - 4/14/2014 and he's 100% perfection!!