The most useful service I got from the NAS was the hyper-link to this place!
It's not so long ago that the NAS was called the National Autistic Children's Society, and as Fran1 said, this is still very apparent. When I wanted to find a local autism self-help group to join, I found that none of the NAS groups near me support adults on the spectrum. I understand the need to prioritise when resources are scarce, but I also think that they must miss out on autistic volunteers who could have a lot to share if we weren't kept so much at arms length (we all used to be autistic children once, after all.)
I'm also disappointed with their apparent apathy towards social and political issues. Compared to other disability charities, they have raised barely a squeak regarding recent cutbacks to benefits, healthcare and community support, some of which, like the introduction of PIP, have had a disproportionate effect on autistic people. Although it's nominally a charity, the NAS generates much of its income by providing services to government and local authority bodies, so I suspect there is an element of being afraid to bite the hand that feeds them, and over-reliance on the kind of projects which those customers deem most appropriate and publicly acceptable.
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When you are fighting an invisible monster, first throw a bucket of paint over it.