We have one for DD, and its been great. We got it when she was about 4 months old but started using at at 6 months old. She is 18 months now and still uses it occasionally.
PLEASE dont use them on bench-tops and chairs, they are intended for use on the floor, and with good reason. My DD was able to get herself out of hers within weeks, and if they lean backwards/forwards the bumbo will topple over. I know of a few babies where this has happened on a high area and they have fallen and hurt themselves, you cant be too careful!

Chiropractors and the like dont recommend it. We took our DD to a Chiro frequently, and we asked him about it and he said they dont recommend it as it can put their backs out and cause their spines to devellop incorrectly.

Apart from that though I think its awesome, I know of women who used them as bath seats (although this does ruin them) and I found it great when having a picnic!