I have found that some students encounter this problem.
Well, accoring to Oxford Dictionary (http://www.oxfordlearnersdictionaries.com/definition/english/as_3)
as… as… is used when you are comparing two people or things, or two situations
He was as white as a sheet.
You aren't as tall as your father.
She doesn't play as well as her sister.
I haven't known him as long as you (= as you have known him).
not so… (as…) is used in comparisons, meaning not to the same degree
I haven't enjoyed myself so much for a long time.
It wasn't so good as last time.
It's not so easy as you'd think.
He was not so quick a learner as his brother.
so, the difference is: while you can use AS..AS for both negative and positive, SO...AS can only be used with negative.
She is as beautiful as her sister. - CORRECT
She isn't as beautiful as her sister. - CORRECT
She isn't so beautiful as her sister. - CORRECT
She is so beautiful as her sister. - INCORRECT
A tip: if you only use AS...AS for makng comparisons (doesn't matter whether they are negative or positive)- you're on the safe side:)
"I haven't enjoyed myself this much in a long time" also sounds more natural to me.
"so... as" doesn't sound very natural to me. While I've heard it before, it's almost never used in the US. I'm guessing that it's used more in the UK.
Excellent advice, Dasha.
Life is short, and there is no point in making things more complicated than they need to be. We have a perfectly good expression 'as ...as', which sometimes - but not always - can be replaced with 'so....as'.
Given that there is no situation in which you have to use 'so...as', the most rational thing to do is to forget about it. Don't use it at all. Just use 'as ..as'. I was about the offer this as my word of wisdom on this topic, but then I scrolled down to the bottom of your entry and saw that you'd already said this. Most sensible.
If only all our problems could be solved so easily.....
Does accoring or according?