I did become obsessed with Omega-3 fatty acids for a period of time, although I have never taken them myself. There is reliable and thorough research on their usage for depression and schizophrenia and a little research for other purposes.
As someone pointed out, flaxseed oil, although it does contain Omega-3 fatty acids, it contains them in the form of ALA while your body uses DHA and EPA. Your body is able to convert some of the ALA, but not very effectively.
Looking at the studies, it seems that for most treatment, EPA seems more important than DHA in producing positive outcomes (while DHA is easier to isolate).
Researching, I have looked at Eye Q and found that, although it is probably not harmful, it has very low levels of EPA and DHA. The one study that utilized this product used a dosage of 6 pills per day, equaling about 500mg of EPA and 200mg of DHA. Psychiatric studies usually utilize dosages between 1 and 3 grams. As an alternative, I am a fan of OmegaBrite, as it has one of the highest levels of EPA per milligram on the market or PBL Pure Omega 3, which although it is not quite as pure, gives you 100 1g pills for about the same price as most merchants give you 60 500mg pills. With OmegaBrite I would recommend at least 3 pills a day. With PBL Pure Omega 3 I would recommend at least 2 pills a day.
Next, there are several vegetarian/vegan Omega-3 pills available on the market, although they seem to provide primarily DHA, advertising their DHA content and not mentioning their EPA content. I will provide links:
http://www.devanutrition.com/vegan_DHA.html
http://www.water4.net/products.htm
http://www.detoxyourworld.com/acatalog/omega_zen.html
I have been unable to find comprehensive and conclusive studies in the usage of EPA or DHA for the treatment of ADHD or Autism. I can say that it is quite unlikely to be harmful.