Not good vs. No good

no-good adj. [only before noun] (slang) (of a person) bad or useless

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"It's no good" means that there is no effect. "It's not good" means there is an effect, and it's bad.

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The meaning is very similar but it's not exactly the same. As a native speaker I would use them in different ways, but I have to think hard about the difference. I would say that "It's no good" and "It's no use" are used in specific situations where there is no point in doing something. I would say a typical example would be:
"It's no good/use trying to talk to her about it - she won't discuss the situation".
So, in fact, I think they might be synonyms and used when something is pointless, as opposed to a bad idea or useless.
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To me, it's no good can sometimes mean it's not good at all, so in this case, it's no good and it's not good mean essentially the same thing, but it's no good has more emphasis.
smoking is not good=smoking is bad
smoking is no good=smoking is really not good
However, in this case the difference is not very big and could vary a lot depending on the tone of voice.

It's no good can also mean it's pointless, perhaps emphasizing that you've already tried to make it work.
It's no good. I've tried so many times and it just doesn't work.

I think it's no good often means it leads to nothing good.

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The phrase "no good" typically refers to quality or condition.
It can also be a compound adjective. In that case, we would use a hyphen "no-good".

no-good (nō'gʊd')
adj.
Having no value, use, merit, or virtue.
http://www.answers.com/topic/no-good?method=22#after_ad1

The phrase "not good" can refer to quality, condition, or something that one finds to be bad or not pleasing.

Here are two examples:
It's raining, so we can't go to the park. That's not good. - This is something the speaker and the listener find to be unpleasing or bad.

This keyboard doesn't work anymore. Throw it away. It's no-good. Here, the speaker is referring to the condition of a keyboard. (You could say "not good" here, but this is an example of how someone would typically use "no good.)

We could also say "throw that keyboard away; it's not good anymore".

However, I would say that using "no good" in this in this sentence would be less likely "we can't go to the park because it's raining; that's no good". It's not impossible, but I would say it's less likely.

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As shown by two dictionary definitions, we can also use "no-good" to describe a person.
http://dictionary.cambridge.org/define.asp?key=53843&dict=CALD
http://www.answers.com/topic/no-good?method=22

Also, "no good" sounds rather "informal" to me. At the least and at the most, I would say "no good" is a phrase that is best not used in formal writing or when a more formal presentation of language is best. I have nothing against informal language. However, I think this is noteworthy. It has to do with register.

Here, we can see that it is labeled informal in the American Heritage Dictionary.
http://www.answers.com/good

no good (Informal)
Worthless.
Futile; useless: It's no good arguing with them.

http://www.answers.com/topic/no-good?method=22

no good
Unsatisfactory, inadequate; also, no use. For example, This work is no good; it'll have to be done over, or It's no good complaining since there's nothing we can do, or I tried to appeal to his sense of generosity, but it did no good. [Mid-1800s] Also see come to an end (to no good), def. 2; do any (no) good.

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