I've said it before, and I'll say it again: If you're an experienced BWer, capable of throwing your LO on your back with multiple carriers without breaking a sweat, then the Butterfly is not for you.
But, if you struggle with back carries, and need a little more assurance. Or if you liked your Bjorn and need something more comfortable . . . The Butterfly is a perfect choice.
I fell in love with the Beco before the Butterfly (4th generation), and when I got my butterfly, I couldn't like it as much as I wanted to. I felt like it was limited because you always put baby in the same exact way. Also, there seem to be a lot of buckles and straps. I felt (to me) like it took twice as long to put baby in or take him out, when compared to the 4th gen.
That being said, the Butterfly is innovative and a great choice if you are less comfortable wearing baby with out safety measures.