I love the coocon that fits in the pram, it beats buying a pram liner.
I guess the only down fall is they will outgrow it quickly. By the time you want to put sit them upright in the pram you wont be able to use it.
I guess you could still use it for getting out and about when they want to sleep, but you can make do without it if you're having a baby on a tight budget.
I'd again say I do love it, but it's definitely not a necessity.