This. As much as it may work right now, because basically no one is doing it, as soon as it becomes a popular method, it seem to me it will stop working pretty quickly. Everyone doing it will become equivalent to a Jehova's Witness knocking on your door.
That said, I imagine I'd be more okay with it if every time I said yes I got a free coffee.
Aside: I wonder how different reactions would be in different countries/cultures.
In Mexico people try to trick you by asking something for something innocuous, like the time, and then ask you for money.
Also a supposedly common scam is to get personal info such as names and phone numbers in surveys and, later, to use this information to make fake kidnapping calls sound more plausible.
The upshot is that you learn to ignore people who try to get your attention. I don't think it would work too well here.
On that note, I'd imagine that "don't ask for ANY personal information except an email address, right at the end" is a pretty good rule for this sort of testing anywhere in the world. Good point!
The technique is somewhat self-limiting. Unlike, say, email spam or online annoyances, you've actually got to be there in person for it (though sufficiently large organizations could hire bodies for the purpose).
Still: the method has significant costs. That's a good think in the "preventing overuse" department.
To me, what made what he was doing appealing was that he wasn't directly soliciting people. He made the arrow and the sign. He wasn't harassing anyone in line. I think that makes all the difference.
This is kind of silly. Doing street level market research is not a new idea and approaching people directly isn't some huge revelation. There aren't so many entrepreneurs in the world testing ideas that you would ever be affected by this more than once.
Don't be afraid to approach people and ask for their help! Most people would love to know that someone cares about their opinion. There are huge online communities that run entirely off this principle, like Yelp or Quora. If someone's not interested, they'll tell you.
This is fine on the street, but not inside a Starbucks. As soon as you start approaching customers inside a business, they have the right (and obligation) to ask you to leave.
The OP had a table set up with the gift cards, and was only (I think) engaging people who came up to the table. Presumably he had the permission of the shop management to set up on the table. I'm happy with that provided he wasn't hogging the big table at peak times. If you are not interested, just don't go to the table.
I'd hate people walking up to me at random inside the coffee bar (UK reserve perhaps).
If many people did this, it would quickly become an incredible annoyance. Have some respect for the commons!