I don't have any disabilities and I Have not known anyone with any for any significant length of time so I can't really contribute.
However, one question I have to ask (and I understand I may come under fire for this, but please be gentle) is whether a lot of the people who have been diagnosed with ADD have problems focusing for extended periods of time doing things they enjoy, such as videogames, art, etc?
I ask this not because I don't believe in ADD (I haven't studied it any way significant enough for my opinion to be anything but personal) but because I see a lot of people who say they suffer from ADD who have no problem spending large amount of time focusing on videogames, painting tiny intricate models, and even programming on C++.
I personally believe ADD exists, but I also believe that there is a certain proportion of people (not sure how much) who use it to cover their own, or others, poor behaviour.
I have trouble focusing if I don't feel like doing the activity and tend to put it off, but I wouldn't say I have ADD by any stretch because I played Mass Effect 2 yesterday for six hours while reading every Codex I obtained along the way.