We shouldn't have talked to them, but as I said, we didn't know what we were doing. One of them basically had us reverse engineer our stuff through demos for six months and then abruptly ended contact. We heard through the grapevine they had decided to develop their own version, which came to fruition. The takeaway is they probably did this with a few of our competitors and simply took the good stuff. A lot of awful, sleepless nights working with their tech team under the misguided impression we'd see a nice exit.
The next one came not six months later, another one of the big (although aging) ones. As part of the dating process they gave us free access to a bunch of their APIs but nothing else happened or happened incredibly slowly. The takeaway there is they weren't serious, they just wanted to keep us on the hook.
When we talked with VCs later (yes, later), they laughed at our naïveté and offered essentially the same advice - don't do it. Don't do it unless someone is talking numbers, very, VERY soon after the MNDA.