This very much depends on your "public". Getting early feedback from the people who in the target market for your product is awesome and stupidly useful.
Paying lots of attention to the people who aren't your target market can be incredibly misleading and harmful.
I've seen all to many products where they've ended up building things for other designers (or developers or whatever) rather than they're actual customers. Which have then crashed and burned.
Thanks for sharing, Jonathan! If anyone else has a good example of other startups building in public, share here. :)