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.
"While teachers of the middle way, mind only, transcendent wisdom, mantra, and other schools may have their own assertions, the fulfillment of those intentions is the same. There is not a single thing that is not contained within mind."
(Gampopa to Düsum Khyenpa, in "The First Karmapa", KTD Pub, p254)
“If you recognize the world of appearance and existence as the mind, realize the mind itself as empty, and have no grasping at the superiority of your realizations — this is the ultimate view."
(Chegom Dzongpa, in "The Book of Kadam", Wisdom Pub, p609)