Do I think they are a good idea?
Of course. I remember for the longest time whitehouse.com was a porn site. (it might still be...) But now if an adult business has whitehouse.xxx, I think we'd all know ahead of time before we clicked.
Not the best example, but you get the drift. Surfers will have a pre-warning.
I guess it could swing both ways for the adult biz, but I'm one for organization and for classifying everything. If I owned the internet starting with media, I would have all music related sites like madonna.music. All movie sites: terminator.movie, etc...
But that's just me.