Saturation is a perfect word - it describes the ways in which, in any religious group, there is a guaranteed hard core of committed believers who are passionately involved.
I don't think, though, it necessarily speaks to any cultish elements of the JWs. Any group can have over-the-top members. Ever been around hard-out zealous fundamentalist Christians loudly selling their message on street corners? Whoa - let me out of their sight! Yet they too claim to "just" be preaching the gospel. Yeah right!
BTW, what interests me much more than the fairly predictable small core of absolutely committed believers is the ever growing pool of nominal believers - this is perhaps one of the most interesting, if not striking, developments among JWs in recent years. The hangers-on grow ever greater in number while that small core of zealous devotees gets smaller and smaller...