Edited: See the bottom section!
It isn't challenging the response. It is asking for more information. I accept the answer of "good" and then ask for elucidation. The follow up question ("really good or just sorta' good?") is predicated on my acceptance of their response. If I were to continue the questioning beyond the initial request for more detail it would definitely be intrusive, especially coming from an arbitrary stranger.
If you were to respond to my follow-up question with anything other than a positive reaction I would not attempt to engage you in any meaningful way beyond that. It hasn't happened yet, but it is entirely possible that it would!
Edit: I owe you an apology. After a little bit of thought (I posted shortly after you without giving what you said the consideration it deserves) it definitely could be construed as questioning your initial response of "good", which would be rude. I just had my understanding improved a bit. I hadn't even considered it that way (even after you spelled it out)!
There are other e-readers that have far less stringent requirements for getting books. The Nook and Kobo are an example (as are the Sony E-Readers). I have a Nook and have yet to purchase any books from the Barnes and Noble store. I constantly put DRM free books from Project Gutenberg on it and just placed the Less Wrong sequences on it as well. There are also FLOSS programs for editing PDFs to make them easier to read on an e-reader. A little research goes a long way!
The phrase itself is less important than the "stepping outside of their hum-drum routine." If you can get them to engage in what was going to be just another surface level customer interaction it can be a good thing.
Of course, that question is not the only one I use, but it was an example of stepping outside the expected script. An unexpected, but apparently friendly, response tickles the right places in many people.
I have replaced the stock replies to normal social banter with something just on the edge of what most people expect. That little change has had a positive impact on my everyday life. When you ask someone how they are doing they will usually respond with the standard, "I'm Good". A simple smile and a, "Are you really good, or just sorta good?" tends to bump them away from the script and engage with you a little more. Whether it's a waitress or a mechanic, that simple statement (no matter how scripted it is on my part) tends to bring out a higher level of service from them. There is no wasted energy in trying to "connect" with them, as I usually don't care... but stepping outside of their hum-drum routine gives them the perception that I care. That can make all the difference!
I rarely do movies by myself, having recently gained a permanent significant other. Movies can run anywhere from $9 (Matinée) to $16 (IMAX 3D). It is a travesty, but the experience is often worth the cost. Sneaking in candy or drinks can make the outing that much more exhilarating!
On the opposite side, and also worthy of discussion: How NOT to Argue with Religious People, Conspiracy Theorists, and Other People Who Believe Crazy Things.
I recently began to use a similar method and have seen my own lack of thought when it comes to what I am willing to spend my hard-earned money on. I will easily spend $25 dollars to see a movie in a theater (a one time viewing) yet hesitate at spending $19.99 on a DVD I know I will watch several times. I will unwaveringly pay for dinner at a restaurant, but stand, with great time spent in deliberation, in a grocery store aisle debating the cost of ingredients. I am slowly gaining a better sense of perspective. Keeping track will do that!
I am very new to the site, and have, in the short time I have been here, found it to be both a pleasure to navigate and easy to use.
Although I could very well fit under the category of "advanced user".