I happen to love "the butterfly bush." In fact, it's one of the plants that first amazed me before I knew a thing about plants and gardening. Here in Portland they grow like weeds-which unfortunately, is what many people think of them as now. Several years ago B. Davidii was added to the dreaded noxious weeds list here in the Pacific NW.
For a time, I jumped on the anti-buddleia bandwagon and discouraged people from growing them because of their invasive nature. But now I've chilled out and I feel differently. If you live in the city and you prune the darn thing before it starts to go to seed I don't see an issue with it. If you live near a wetland or an agricultural area and you have an acre filled with 15 foot tall buddleias who are spraying their seeds far and wide you may want to reconsider.
All this said, I would still avoid B. Davidii-not only am I tired of looking at it, it is the most invasive of the bunch. Instead try B. Globosa, B. Asiatica or B. Weyeriana as more "politically correct" and more unique alternatives. Asian Moon is a new sterile and therefore "guilt free" variety if you want that traditional lilac color.