I don't think is very useful.
Too small and very soon, babies can't fit inside!
Furthermore, once baby can sit without support, s/he won't sit still inside it. I prefer a sturdy and easy to maintain highchair.
Pros
Allows baby to sit up and see what's going on at an early age.
Easily washable.
Portable and lightweight.
Cons
Baby may not enjoy the seat once he or she is mobile.
Older babies can arch their backs and tip out of the seat.
Some packaging shows Bumbo Seats being used on tables or counters, which is not safe.