"That is a good one!"
I understand. I wanted to point out the fact that it does not make sense speaking wise. French is my native language so it is a little irritating when Duolingo says that I am wrong because of the translations that were provided for the website.
I know how you feel. That is why it's nice that there is a button on each exercise for reporting problems that you can select "My answer should be accepted" from the list. The website is always improving based on user input so eventually it will be quite proficient!
What exactly is the difference between: Il est bon celui-ci ! Il est bon celui-là !
Not much, "ici" and "là" are rather interchangeable in everyday conversations.
"ici" and "là" yes, generally interchangeable, even though they techically refer to "here" and "there" but "-ci" usually refers to "this" or "these" whereas "-là" refers to "that" or "those".
It does not make sense. If you say "c'est une bonne" you have to have something else after bonne since it is an adjective. C'est une bonne pomme, poire, voiture, etc. "il est bon celui-ci" would mean He is a good one and not this is a good one. Let me know what you think about that.
You are definitely correct on "C'est une bonne". In fact this phrase can mean "That is a maid/servant". See http://fr.wiktionary.org/wiki/bonne for that. Keep in mind that "il" and "elle" can always refer to "it" based on the gender of the object in question. Generally if there is no gender present, the masculine form "il" is used. Now if we translate "Il est bon celui-là" word for word, it becomes either "he is good, that one" (requires a person present in the context) or "it is good, that one". Though, since we cannot translate word for word for intelligent speech, the second one turns to "that is a good one".