I would buy it, if it was something that solved a problem in my life, but Arduinos are really big and expensive ($30) in comparison to the ATmega16U ($3). Including one really changes the nature of the product, especially for consumer goods.

I would support instead a board with a surface mount atmega and well labeled pads connected to all of the unused pins, especially the programming pins. If you wanted to make it really hobbyist friendly, include a micro-usb connector and put the arduino bootloader on the chip. Doing this would also eliminate the huge USB-B connector, DC jack, and header pins leaving you with a completely SMD board. You most likely would be fabricating an additional board anyways (as an Arduino shield) so there is no extra cost for this approach and you can shrink your board thickness by ~0.5" (as well as the other dimensions if you don't need the extra space)

That said, if you were selling project kits to Arduino owners, it would make sense to stick with that form factor.

Couldn't you get the same benefit by just using the Arduino bootloader and including a USB connection? That way things could be reprogrammed in the arduino environment and you could still use your custom PCB. If nothing else it would be much cheaper, and you wouldn't be constrained by the arduino's size/layout.

Agreed, it would be cheaper, and there wouldn't be size/layout constraints - but I'd rather let people continue to hack with the products, and for that reason, it seems to make more sense to me to keep the Arduino in the picture.

I totally would. I just got a MaKay MaKey because it's a cool thing that lets me tinker around a little and do some fun stuff, without the commitment of having to get a ton of soldering stuff etc. But I know that I now have an Arduino which I can reprogram at will.

I would. Especially the thought of certain products being extensible beyond purchase. I'm in the process of contemplating doing something similar leveraging platforms like Pi, BBone, and Arduino for different collaborative components.

I would purchase a product I needed or wanted, that satisfied the need/want in a quality way at a reasonable price.

Arduino under the hood would make the product somewhat more attractive; I can hack it to do whatever I need it to do.

Is this just about being able to reuse components beyond the original use case, or also being able to split the product and let hackers drop in existing Arduinos we have laying around?