I agree with all the sentiments about it being "developmentally inappropriate". I mean, if they're ready to sit up, they'll sit up! I've never really had a good feeling about propping her into positions that she was unable to stay in, so the bumbo falls under that category.
HOWEVER, we own one, and have used it. In fact, I got roasted for saying this last time - my husband travels for business, and my friends and family live 2000 miles away. I don't have a huge (or any support network), so being self sufficient was a huge goal for me. I simply cannot wear my daughter for everything.. as much as I'd like, and the bumbo bought me a few moments to make myself some dinner or pee when hubby was not around to help.
I think its life span around here was a matter of weeks - she didn't want to lay flat and couldn't sit up. Perhaps not "developmentally appropriate", but still useful none the less. I have every confidence that women have been propping babies into "unnatural positions" since the dawn of man if it meant they could pee without Baby screaming and scaring off a herd of woolly mammoths or buffalo.
So far, for the few hours, cumulatively, that she has spent in it, we have seen no detrimental adverse effects.
Weigh the risks and benefits. If using it has more benefits then not using it (such as being able to feed yourself or not mess in you pants, as was the case with me) then it might be a useful tool.
FWIW, we now use a high chair or play mat, which is much more "developmentally appropriate" as she is in a natural sitting position.