For the most part we found people weren't actually interested in taking any money for coffee (we didn't go the gift card route, we just approached people in line directly). When we offered to pay some people actually found it a bit off-putting. That said though, when you approach people candidly looking for feedback on a project you're personally invested in, people seemed pretty generous with their time.
We ended up getting great feedback on everything from the usability of our website, to the desirability of our product and pricing. We even ended up finding a couple new customers along the way. I'd highly recommend this experiment for any consumer facing product, it can be a bit daunting at first, but when used strategically it can be far more productive then having your head down coding in isolation all day.