The idea of a "witness" is not Chan, if anything at all, a mistake of Chan. As Foyan Qingyuan said,
People nowadays mostly take the immediate mirroring awareness to be the ultimate principle. This is why Xuansha said to people, "Tell me, does it still exist in remote uninhabited places deep in the mountains?" (Instant Zen, p. 107)
I should have put a hyphen or a comma there to make it clearer.
And to answer your question: it is both something I tried to use as a practice and also something I have an opinion about.
"There is no such thing as the real mind. Ridding yourself of delusion: that's the real mind."
(Sheng-yen: Getting the Buddha Mind, p 73)
“Don’t rashly seek the true Buddha;
True Buddha can’t be found.
Does marvelous nature and spirit
Need tempering or refinement?
Mind is this mind carefree;
This face, the face at birth."
(Nanyue Mingzan: Enjoying the Way, tr. Jeff Shore; T51n2076, p461b24-26)