We have used this in various flights the passed 2 yesrs in Europe, US and Asia.

It has been much of hit and miss. Most peopel (travel agents as well as gate agents and crew) still do not have a clue what this means. Sometimes, as mentioned before, we have been approached and asked what assitance we need, in other cases (in asia) this has been equal a wheelchair or just ignored.

My experience is that it is worth having it in the reservation in case something happens. If for example flights are cancelled the note in the reservation usually allows priority in any rebooked flight (happened twice).

For example our UA LAS-ORD was severly delayed and while everyone was forced to wait kindly, they did re-book us to an AA flight. The ability to not be able to wait is one if the main issues in autism and I assume the fact that after 2 hours waiting he started to hit the gate staff, scream, spit etc could also have played a role in their decision to "ousource" us to another airline