A friend lent me one for my first child and she fit into it until she was almost 9mo. She loved sitting in it. I bought one for my 5mo son last month and it is a very tight squeeze. Not sure if the dimensions have changed since the originals or my son has a particularly large behind (he is 25th percentile weight). I have had to remove the seat belts as he looked so uncomfortable in it. The marketing says that the Bumbo is suitable from 4 months but I think my son has almost outgrown it at 5 months. He is jammed in so tightly that when I lift him up to get him out of it the Bumbo stays stuck to his bottom! The tray is a nice idea but getting it to clip underneath is really tricky with the baby in the seat. My son is too big to slide in if I attach the tray first. Not sure I would have bought it if I had known given that my boy isn't too keen on sitting in it.
This is a good quality product; however, it hasn't really proven to be that great as it doesn't really accommodate the width of my baby's thighs. It just meant I had to prize him in and out when I used it. This product would probably have been of more value to me if I bought it when my baby was probably around 3 months old as by the time I bought it for him (~5 months) he was only weeks away from being able to sit up on his own.
My baby never fitted in this. He tried at 4 months and it was too tight on his legs even then (he's not a big baby). Tried again at 6 months and after stretching it to accommodate his legs, he as able to climb out... Albeit with his poor thighs stick fast! Literally never used it.
I have a baby come to stay every other weekend. I bought when he was 3 months old so I didn't always have to hold him - it was a little too big then but now at 5 months he fills it out nicely and he's quite big but quite skinny. I nearly didn't buy because of the bad reviews, but I'm pleased I did. I don't struggle to attach the tray, even when he's already in it. He's got chubby little thighs and while its snug, it's not too tight.