I remember interacting with him when he visited the FTII Pune in June 1991 for a screening of his documentary Siddheshwari (starring Mita Vashishtha) for those attending the Film Appreciation course.
His debut film Uski Roti (1969) brought him to the notice of critics and connoisseurs of fine cinema. This film is also well known for the fact that the legendary Malayalam director John Abraham was the Assistant Director.
I remember seeing his film Nazar (1991) in which a forty year old antique dealer (a role essayed by Shekhar 'Elizabeth' Kapur) is obsessed with a teenager (played brilliantly by Mani's daughter Shambhavi). I doubt anyone in the Hindi speaking areas barring cineastes remembers this 'heavy' Hindi film.
The documentary Siddheshwari on the singer - a role played by Mita Vashishtha - was exquisite and I consider myself lucky to have seen it in a hall where he was also present. The Wikipedia article on him has this quote by him "The dividing line from my films and documentaries is thin. Some of my films like "Siddheshwari" are like poetic documentaries." Will Hindi cinema get another director like him? I doubt it even though there are some very promising guys there today.
In her tribute The Solitary Artist written for Hindustan Times the critic Aruna Vasudev talks of his love of Hindustani Classical music and also of painting. According to Aruna he was about to start work on a feature film based on the book Under Her Spell by Dilip Padgaonkar on Rossellini's stay in India during the fifties when his affair with an already married Sonali Das Gupta scandalised India and Nehru (of all people) had to ask him to leave.
Click here for the Wikipedia article which also contains plenty of links to articles on Mani Kaul.