I bought one for DD2, never had a need for DD1 and certainly would not have needed it with her.

DD2 on the other hand was raring to go from the word dot and was such a cranky pants if she wasn't sitting up or moving or doing whatever it was she was wanting to do. I invested in one at 5 months for some relief from her frustration and it was great. I only used it for a couple of months with her then it became redundant as she was too keen to move or sit on her own but for that short time, for me, it was well worth the investment.

I have since lent it to a friend with twins and it has certainly come in handy when there are not enough hands to hold babies!

I don't think they are for every baby and I was aware of the controversy surrounding the use of them when I bought mine (they are not recommended to be used for long periods of time as a baby is not designed to sit up necessarily before being developmentally ready, google with help flesh that out a bit ) but for my situation with DD2, it gave me some sanity. I just kept in mind the discussions against them.