I would suggest there is a subtle difference in meaning. Tolkien and Warsaw Will are on the right track here.

"He's not as successful as his sister" simply means the sister is more successful than the brother.

"He's not so successful as his sister" similarly means the sister is more successful than the brother, but also means that the sister is very successful. The "as" version implies nothing about whether either is particularly successful.

if "so" is less common, it's not necessarily because it's wrong; it may simply be because the difference in sentiment it conveys is not as often needed or intended.

According to Merriam-Webster Dictionary of English Usage, the "not so ... as ..." used to be the regular form in negative comparatives; it's "as ... as ..." that is the relative newcomer (for negatives). Jane Austen, while regularly using "as ... as ...." in positive comparatives, used "so .. as ..." in negative ones:

replace 'long' with any other "adjective" such as 'big', 'hot','ugly'.as "adjective" as - is a comparisonso "adjective" as - does not make sense . ( So big , so tall...so is synonymous to very)If you explicitly use the phrase 'compared to', then it makes sense."The new Severn Bridge is not so long,compared to the old one"OR "The new Severn Bridge is not so long" ( compared to nothing or in general or based off the context of a previous sentence , eg: "The old bridge is long".)

"The new Severn Bridge is not required as long as the old one does not fall"Replace the "idiom" 'as long as' with 'so long as'"The new Severn Bridge is not required so long as the old one does not fall"

It is difficult to find a difference between 'as long as' and 'so long as'. Both are inter-changeable. Still I often feel, while talking 'so long as' is used more than 'as long as' which is common in speaking and also writing.

Growing up in Sydney in the 1950s and 60s, I rarely heard 'so long as'. 'As long as' was much more common. However, these days the reverse seems to be the case. The phrase 'so long', an informal way of saying 'goodbye', was very often heard in those days, especially in American TV shows and movies. Is it possible that the increase in usage of 'so long as' is a sub-conscious extension of 'so long'?

As nearly everyone has already said, when they mean "provided that" in a conditional - they are interchangeable.

But I don't quite agree about comparisons. In "as ... as" comparisons; "so" can replace the first "as" after "not", at least in BrE:

'After not, we can use "so ... as", instead of "as ... as". This is structure is more common than "less than" in informal English: "He's not so/as successful as his sister" '(Practical English Usage - Michael Swan - Oxford)

So although I think I would usually say "as", it's quite possible to say "I haven't been here so long as he has". But in positive sentences only use "as".

I find the variation useful for moving the stress in a sentence, since "so long as" is the less common. Compare:

"The new Severn Bridge is not as long as the old one"

with

"The new Severn Bridge is not so long as the old one"

The first version seems to be making a simple comparison of size, while the second seems to be making a qualitative observation about the length of the old bridge; without the context we can't tell if the speaker feels that more length is better or that the old bridge was over long.