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.
Solutions like cardboards are something would harm vr in long term. Because people will try them and think this tech is sucks and will avoid vr systems at all.