Here's what's great: the playing of the VPO, the sound, some of the individual performances, and the depth of the casting. You'll see that people list Gwyneth Jones and Helga Dernesch as participants. Well, Jones is a Rheinmaiden (!) and Dernesch is a Valkyrie. Can you tell the Valkyries apart or even follow any of their individual lines without a score in front of you? I believe it makes a difference to get the best singers you can find for the smaller roles - this was audible, for example, in the recent Met Boris, where you didn't just have strong singers at the top of the cast, but into the small roles. Nearly everyone came from the first rank of Russian singers, in fact. At San Francisco, we get the Adler Fellows in most small roles; our Rheinmaidens are all prominent singers, but many of the Valkyries are Adlers.
The Solti Ring was and remains a landmark because of the care taken with the sound quality and the many sound effects Wagner calls for. In truth, they're not all great - these days, we're so used to fancy sound in movies that fall of Gibichung Hall sounds like a pile of Legos collapsing. Still, the six harps in Das Rheingold, the mystery of who is singing Siegfried in the horrific scene where he steals Brunnhilde from her rock, and the general gorgeousness hold up beautifully a half-century later.
However, there's a big caveat about the sound: the CD transfers of the Solti are uniformly mediocre. They do not accurately reflect what's on those LPs. The first release was fairly early in the days of CDs and it is harsh and overly bright, which is not how the LPs sound at all. For the fancy second release, the one in the nice cardboard box, it seems they didn't do much to mitigate that, if anything. A friend who has both and the LPs guessed that the original tapes were damaged and/or London/Decca wasn't willing to do a full new transfer. Honestly, this set should be on SACD, not regular CD. But it's not.
How you feel about the set as a whole might depend on how you feel about the individual singers, or about Solti. Now, Solti has his moments, and sometimes they last for a whole opera. Rheingold and Goetterdaemmerung are both (mostly) splendidly conducted, though he falls down and lets things slacken in some of the contemplative moments. Die Walkuere is sadly studio-bound and unspontaneous sounding. Siegfried is excellent.
About the principals. Well, Hans Hotter, a stupendous Wotan in the 1950s, was past his best; by the time Walkuere was recorded. It was the last of the set, he was in his mid-50s, and he had developed a noticeable wobble. If you want to hear him at his best, get Keilberth's 1955 Bayreuth Ring or Kna's from 1956 or Krauss's from 1953.
Me, I am not a big fan of Nilsson. She didn't record well, and far too often sounds to me steely in tone, vague in pitch, watery in her middle and low register, and unnuanced in her approach to the text. I am not the only person with these doubts about her; see, for instance, John Steane's The Grand Tradition.
Then there's the Rheingold cast. Culshaw was hot to have some continuity with the past generation of Wagnerians, and he also wanted a younger-sounding Wotan than Hotter. So we get Kirsten Flagstad, who was then age 63, and George London, who was in his brief prime. And in general, if you read about this set, Flagstad is highly praised, with people talking about the miraculous preservation of her voice.
PEOPLE. Are you using your ears? Flagstad sounds nothing at all like her gleaming, golden, younger self. The great voice sounds thick and immobile and decidedly aged; if you want to hear the true glory of Flagstad, go to her 1930s and early 40s recordings, especially the many live Isoldes that are out there. The effect isn't at all what Culshaw wanted: she sounds like London's mother, not his wife.It's too bad they were so keen on finding a role for Flagstad, for the set would otherwise feature Ira Malaniuk, a superb Wagner mezzo who can be heard in numerous roles on several 1950s Ring sets.
So the set starts out with a few big strikes against it. I'm fine with Wolfgang Windgassen's Siegfried (and of course there's the famous story of how Solti and Culshaw wanted a hot young tenor who just didn't work out because he couldn't or wouldn't learn the part) and many of the other principals are tremendous - Ludwig's Fricka, Neidlinger's Alberich (one of MANY), and so on. But it's not the greatest thing since sliced bread, and it's not my first choice to listen to.