No, these used wrongly, or "I am so SO" (with the wrong stress to boot) often rub me up the wrong way too. You might say "My English is not too good" if your English is in a sorry state and you feel rather sad about it. It doesn't mean the same as "My English is not very good", of course, but it's a possible statement. "Not so good" is what you might say if you want to break bad news to someone gently, for example.

Thanks, CM2DD. I like the example with "The news is not so good."
But how on earth do you explain these nuances to Germans?
Oh well, I don't expect you to answer that one - it's purely rhetorical!

I'll try:

Not too good - current state, may change soon
Not very good - general state, no change likely in the short-term
Not so good - "not as good you may have hoped for" - referring to an anticipated outcome

How would you explain the differences between the following
constructions to English-speaking people?

- nicht so gut
- nicht sehr gut
- nicht gerade gut