I found the Bumbos absolutely brilliant as I started putting my little girl in hers at about 2.5 months. She is a week off 8 months but no longer likes it as she can crawl around and this restricts her. I wouldn't recommend using the bumbo once they can move around or lean over it ,as although the base is wider than the top, I think she would be able to topple it. I would reccomend from about 14 weeks until 7-8 months.
Everyone I've asked who used one also says they stopped using it at around this time. Maybe Bumbo can't justify £30 for a short space of usage time but I'd still recommend using it. It made life so much easier at times. I used ours to feed her in, she sat at the table with us and was made to feel part of things, when I was in the shower she was in her bumbo outside the bath at the bottom so I could see her and she could still see me, we used it in restuarants, for picnics, in the garden when hanging the washing out.....the list is endless.
Please don't be put off by bad reports but I would suggest to Bumbo that they cut the amount of months that it's suitable for.