There was an article in today's paper about children suffocating in their car seats. It's a little scary since car seats are so convenient with small babies. You strap them in get them in the car, and then they can stay in the car seat when you're running errands or even after you get home if they've fallen asleep- which is pretty common.
The article suggests only using the car seat in the car, even if it means waking a baby that is napping.
My second son sat in his car seat a lot. I was busy with his older brother, and he was a really mellow baby. He would just sit there looking around, so until he got fussy, I let him stay there. I guess I won't be doing that again. It's a good thing for another reason too, son #2 has a flat spot on the back of his head. The dr. kept saying it would pop out, but it hasn't. Maybe less time in a car seat is beneficial for head shape too.