Good question!

There is something about the phrase "good question!" which sends my blood boiling. That's because mostly it is another way of saying "I don't know the answer to that one". Since few people have the guts to say "I don't know", "good question" is here to stay. However, I must admit, it is one of those old tricks that still works. While the questioner is gloating over his success, you can think of a way out. Or, at least you have him on your side. The interviewer is no more the fanged panther after a dosage of "good question" has been administered.
Rarely would you come across people who would combat "good question" with a "Thanks. Will I get a prize for it?". Perhaps a true bred Haryanvi will have the guts to do it, but not ordinary mortals like us.
"Good question" is also used sometimes as a precursor to answering questions that you know like the back of your hand (Just as Miss Worlds know the answer to "If you were reborn, what would you like to be born as?"). Here, "good question" is just another way of saying "Thanks for giving me a chance to appear extremely learned. Let me augment your reputation a bit too".
The truly good question is one that twists a problem such that it is easier to solve it. Lessons can be learnt from the oft-forwarded anecdote where NASA spent millions of dollars on a ball-pen which would work in zero-gravity. A good question like "What do we need a pen for?" would have made things much more easier.
Q: "What do we need a pen for?"
A: "To write notes"
Q: "What else can be used to write notes?"
A: "Chalks, pencils, sketchpens, markers and even PDAs to some extent"
Q: "Whats wrong with these?"
A: "Ummmmm...well, I think pencils would be just fine"

Note: Nevertheless, I must confess, I have a liking for the other variant of "good question":- "it depends". One, because you can always ask back: "Depends on what?". Two, a lot of things in life actually do depend on hundreds of factors. So, you are not lying at least.