Meh. I've got high-functioning autism (diagnosed as Asperger's, but if I recall correctly, the two were merged. I dunno, I'm not an expert on these things), and it can affect my life to a significant degree at times, like when I feel an urge to talk about/think about/read about/etc. dinosaurs or Pokemon. At times, such as when it is too noisy, I cannot focus, though some of that may be due to my ADD.
As for the misuse, I don't get it too often, but it's kind of irksome, especially when low-functioning kids are the stereotype for autism, and they get most of the attention for it, leaving some of the higher-functioning kids without help. Self-diagnoses aren't too good, but I really don't care much, just so long as they don't take it too far.