Good comments everyone.
Something I would like to add. I think that some may be confusing what constitutes a religion going "mainstream".
During the Rutherford era the organization was confrontational and hostile with society. Open protests on street corners and heated rhetoric against people (by name!). Read the article Armageddon, Inc. that appeared in the Satruday Evening Post in 1940, that should give you an idea of this.
Eventually the hostile elements faded somewhat, and Witnesses became more integrated with the society they inhabit. Sure, they are insular, but not completely. They do not live in compounds (at least not all of them!).
Going "mainstream" imo should not be put within a black/white framework, where all changes are seen as either positive or negative. The organization and its members are far more apart of "this world" then they want to believe. While elements of the movement deny personal freedom and as some argue create personal injury, this doesn't detract from the fact that the group is loosing its exclusionary nature. In fact, the ever diminishing role of prophetic significance has probably assisted in this regard.
In recent years I have tended to back away from some of the more intense rhetoric regarding "cults". That the social environment created by the JWs is intense and limits personal freedoms is without question. This in turn can create negative consequences for individuals. However, far too many people attach the label of cult to something that the do not like, a factor that diminishes the usefulness of the term. Even worse, I believe former members of religious movements sometimes use anti-cult rhetoric to the extreme, using it as an excuse to justify their previous way of life. The argument that "I had no control, my mind was being controlled" just doesn't cut it for me.