[bovik]

"one helped me out without intending to by talking about why one feature probably wouldn't work without a tweak because they found out it doesn't work without X."

Didn't you just disprove your whole point there. Your competitor revealed his/her ideas about how to solve a problem (that you hadn't even foreseen yet going by your description) and in the process hurt him/herself in the marketplace. Your advice of meeting and talking openly with your competitors didn't really work out for that person.

[brackin]

No because they also received value from me explaining what we're building and he was very interested by it. They are a much bigger market-leading competitor so it's not a case that people will deal with every day.

I can understand why someone wouldn't want to talk to a competitor but not when dealing with normal founders, users, etc. As you pointed out it can be risky but overall being open usually leads to a net positive.