I'm speaking only for mental/neurological disorders.
IMHO: 2% potentially helpful, 98% potentially harmful.
While it's great how much there is out there when it comes to understanding and putting a name on things, I think it should ultimately left to professionals. Yes, no one knows your body/mind like you do, there's more to diagnosis than 'I read a list of depression symptoms and I think I'm depressed'.
"But wait!" - I hear you cry, "I can't afford to see a professional!" or "I live with my family/am underage/live in the middle of nowhere/can't get to appointments and THAT'S why I self-diagnose!"
I can't offer a fix to all those problems, but here's some advice. I can say that a lot of psychologists (at least the ones who don't take insurance) sometimes are willing to talk about their fees and agree on a lower rate. It also helps if you can take up a time slot that's sometimes hard for a shrink to schedule (such as early mornings). These are especially applicable to college kids since psychologists know you're broke and can possibly arrange your schedule to fit that 9am session. Plus a lot of clientele is by recommendation and word of mouth so a psychologist building a rappot is a great thing. And even if Dr. $ can't treat you, maybe they can refer you to Dr. ₵ .
Back on track, I don't put much stock into self-diagnosis, especially 'hot' ones like autism. I understand not everyone can get properly diagnosed, but I also think if it's a big enough issue, you'll do something about it.
I tend to also notice that self-diagnosis often goes hand-in-hand with a lack of trying and self-pity. Like, 'oh, I'm self-diagnosed with anxiety disorder and I can't make phone calls so I can't function and this world is so mean and unaccommodating'.
I just don't think it's a healthy practice and shouldn't really be encouraged.