I have one and used it from 3 months onwards, it was very useful as DS was too small to fit into a highchair, so whilst we would have dinner we sat him in his bumbo on the dining table with us (obvioulsy you would never leave them alone in a bumbo on a high surface) and like meals said its useful when you need to get things done and need them to sit somewhere for a while. Of course now that DS is 9 months we don't use it as he can easily climb out of it. I think it also helped with learning how to sit and strengthen those muscles.
I used one for my previous 2 DS's and I have one for my newest DS already - they're fab! I loved them, I found them great for improving their posture, encouraging the development of their "sitting" muscles and there is only so much a LO likes lying down on the ground before they want to be up - great when you're making dinner and don't have hands spare to carry them!