Yea, brain fog is a problem with some. I figured it's still worth a try, all sides were gone when the users discontinued, so I figured I could just discontinue if I got any unbearable sides.
I'm on the standard 0.5 mg, I was on 2.5 mg for about the first 2 months. The only reason I dropped down was the cost. 2.5 is shown to regrow more than 0.5.
What you heard about Propecia is correct, after ~2mg it makes no difference. Duta is the same way above 5mg (although there is only a VERY small difference between 2.5 and 5.0).