In addition to being able to appreciate two complete acts in a live performance, we have a magnificent Tristan from Suthaus. Listening to his contribution here, one can imagine that this voice is what Wagner must have had in mind for a Heldentenor. His voice is solid, round, weighty and always right on the note. His expressiveness is also outstanding. I have read that Suthaus was a terrible stage actor, but listening to his recordings with Furtwängler one can deduce that he did all his acting with his voice. And this may well be his most expressive performance.
The rest of the singers are also mostly very fine. Erna Schlüter's Isolde is not at the same level as Flagstad. However, it is a excellent performance, also quite expressive. Perhaps the one weakness may be that, somewhat like Mödl in the 1953 Fidelio, her interpretation is a bit too human. Some people may prefer that, I prefer a more mythological interpretation. The Liebestod is outstanding.
Furtwängler conducts the Staatskapelle Berlin, or the orchestra of the Staatsoper Berlin. This orchestra had a great Wagner tradition and still has it under Barenboim.
There is some distortion in a few places, but, in general, the sound is good.