Did buddha said anything about upanishad. Did he consider upanishades teaching use full. Or did he rejected them like vedas.
Only the three Vedas (presumably Rg, Yajur and Sama Veda) are mentioned in the Pali suttas (the discourses of the Buddha). No other Hindu texts are mentioned.
It so happened that a brahman student named Kapadika was seated in the assembly: young, shaven-headed, sixteen years old, a master of the Three Vedas with their vocabularies, liturgy, phonology, & etymologies, and the histories as a fifth; skilled in philology & grammar, well-versed in cosmology & the marks of a great man.
According to Wikipedia's scholarly dates, the Vedas and the earliest Upanishads (Brhadaranyaka, Chandogya) came before the Buddha. Despite this, only three Vedas are ever referenced in the Pali suttas.
A few other Upanishads (Taittiriya, Aitareya, and Kausitaki) and the Nyaya school of logic may have developed around the same time as the Buddha.
If you look at the scholarly dates given in Wikipedia, all Vedanta schools, Samkhya, Yogasutras of Patanjali, Purva Mimamsa school, the rest of the Upanishads (Kena, Katha, Isa, Svetasvatara, Mundaka, Prasna, Mandukya), Brahmasutras and Bhagavad Gita all came after the Buddha's time.
The Upaniṣads were probably composed or in-composition during the Buddha's time, but it can be argued that they were not yet "mainstream" thought. If you go through the dating of the Upaniṣads on a resource like even Wikipedia, you will see some of them estimated to be immediately pre-Buddhist, some are contemporaneous with the Buddha, and some are post-Buddhist. In the EBTs ("Early Buddhist Texts"), the Brahmins are depicted as participating in a henotheistic-appearing religion where salvation is rebirth in the realm of their heavenly father (Brahmins are said to be "born of the mouth of Brahma" mukhato jātā brahmajā in the Madhurasutta).
There are some Upaniṣadic views put into the mouths of Brahmin ascetics in the EBTs, such as the view "so loko so attā" (the self and the cosmos are the same) given in the Brahmajālasutta. All-in-all, in the oldest layers of Buddhist scripture, the Brahmins are depicted with more stereotypically "theistic" doctrines than the more philosphically-refined Upaniṣadic doctrines.
The Sublime Buddha didn't praise any sect and any variant of the many self-view and extremes (exist, not exist, ending, endless...), but nevertheless taught them sometimes, out of compassion. "If they understand only some words, it would be of great benefit for them.
What does good householder think: How many different kinds of views are present under those calling them Upanishades?