well, both my kids got huge quickly, and outgrew the length for those carseats within a couple months, so i never really used it much. and for the brief time that i did use it, it usually just stayed in the car like a car seat.
plus, my kids *hated* being in a car seat until they were old enough to face forward, so anytime they were in them they pretty much screamed their heads off until i took them out. so not something i personally had any interest in. luckily at the time we lived in the city and walked everywhere - it was rare that we drove at all, and when we traveled we did it at night so they would sleep through being in the car. i hope any future kids don't hate the car as much as the first two did - we live in the boonies now, the car is pretty much a necessity if you want to leave your house
i can see where it might be handy if you had a baby who actually fell asleep in it, and were just going to run in someplace quickly, or were going into your house from the car and wanted to let them sleep. my kids just weren't those kids, so i have no experience with that whatsoever.
like others have said, it can be a useful tool, but i think it's very abused.