Droolicious

Products such as this one are not recommended for use in car seats.  There is no actual test standard for aftermarket products and so there is no way to know how they will affect to performance of the seat.  (For example they may put too much padding behind the child and give a false sense that the harness is tight or the way they fit around the harness may affect its ability to move smoothly.)  Use of such products will often void the warranty for the car seat.

For more info see:

babyproducts.about.com/.../buying_car_seat.htm

or

www.cpsafety.com/.../aftermarket.aspx

I agree that they are great for strollers.

Thanks for the info. I added a note in my post.

I disagree about this being a great product.  Although highly recommended on websites, I thought it was horrible.  My newborn son, who was not a "sweater,"  would cry hysterically when places in this thing.  Being a new mom, I had no idea what was going on until I figured out how he was overheating in this polyester product.  I even wrote the company and they suggested flipping it around to the black side, but I had the same results.   Maybe it is great for really cold climates...

We used this the first few months in the stroller, and it was great! We also used it in the bouncer before our daughter had good neck control. Love it!