I know the Boppy is another "sitting aid" but I never found it all that helpful - there wasn't enough all around support to keep the baby from falling backwards. The body pillow, on the other hand, offers high support on all sides if you shape it in a sort of semi-circle around the baby. The length also provides protection for a foot or two either side. While the pillow won't necessarily prevent the baby from tipping over, so long as they tip to the side or backwards they'll hit a soft pillow instead of the floor. Here, allow Moses to demonstrate:
We keep it propped in our papasan chair and it also is a great neck/back support for sitting in there.
I bought mine for an entirely different purpose (floor seating for when friend's are over) but I love that it now serves three purposes (I've also heard its a good side-sleeping aid for when you're pregnant). Considering it only cost me $16 ($10 for the pillow, $6 for the cover) I'd say it was well worth the money. And that makes me and Mose happy.