It is an understatement to say that it is a great loss that mankind does not have commentaries on all principal Upanishads by Sri Aurobindo.
Fortunately the two most important Upanishads has been covered by him in great detail. The most important is of course the Isha and the other of great importance in the Kena. Between them they cover almost everything that one needs to know as far as the Upanishads are concerned. In fact Sri Aurobindo considered the Isha as the most important one in which one can see the foundations of the Divine Life. Sri Aurobindo has touched it in greatest detail as none other. In my view no reading of any other commentary is necessary, except for curiosity sake.
Affectionately,
Alok Da
