Mainstream, not yet. My ordered headset was delivered yesterday.
Unfortunately I had to realize that my smartphone does not have a gyrosensor and a borrowed one from my child not enough power
to look the CosmosCR demo fluently on their website.
But i can already play a little bit with the headset and borrowed smartphones from my friends when they visit me.

If possible do not bother with cardboard, you are going to have a pretty bad experience. Especially if you have a very cheap cardboard headset, the lenses are really bad and it is uncomfortable to wear.
One step up would be a Oculus Go or WMR for about 200 euro.