I will answer this about one of my students
What age was your child when they started using one?
He was 3 when they started using some of the simple talking devices at school, and 5 when they sent one home. (We were already using extensive PECS at this point.)
Were they cognitively delayed?
Yes, he has autism, and possibly mild MR.
What reasons did you have for using one?
He went from being somewhat verbal to having an expressive language regression. Still very high receptive language skills but almost no verbal.
What brand/model did you use? At school, they started with the Go Talk 4, and at home we had the Go Talk with space for 9 pictures. After he did really well with these, we had an AAC evaluation, at which point, they gave him a Dynavox, which is extremely customizable. You can have anywhere from 1-64 pictures on the screen, with multiple overlays and categories.
Did you find it helpful or not?
Yes, very helpful. Remember that in the beginning you'll need to teach the child how to use the device, and while you want to use pictures/photographs that are already familiar, you may end up with a device that has its own set (not good, IMO, but sometimes you get what you get) and then you have to teach those pictures.
Start with a small field of items and add them as the child shows accuracy with his/her choices and comments.
Try to use a recorded voice that's similar to your child's voice. If it's a recordable device like the Go Talk, get a peer to say the items if possible.