I think it's not a real "serious" product; more of a "gag", if anything.
I think it was Google's way at having fun with all the VR hype around last year's I/O conference (and to show it's not anything really groundbreaking).
A bunch of Chinese companies started pumping out Cardboard clones pretty much as soon as it was announced and at least one commercial VR porn company is using them.
We're talking about consumer items, not dev kits. In reality, Google Cardboard was the first (modern) VR kit released.