Be fair, mixture.
You can have opinions on beta software, but they may not be relevant to the released product.
Without end-user feedback from the beta program, which includes opinion as well as problems and bugs (sorry, undocumented features with unwanted effects), the end product as released would not be as good (or would be even worse, depending on your PoV).
I haven't seen Win 10 beta, so I have no opinion.
SD