If you are considering getting this box set, consider no more--it's fantastic. Nothing needs to be said about the quality of Mravinsky's musicianship--he is easily one of the 10 greatest conductors of the 20th century. The Shostakovich and Tchaikovsky symphonies (which comprise nearly half of this set) are authoritative, exciting, and tremendously enjoyable. While Mravinsky may have individual performances of these symphonies that are even better than what's here, the overall level is at a high quality and no apologies need be made for any of them (that said, I am extremely partial to Mravinsky's DG STEREO (not the earlier mono) recordings of the Tchaikovsky symphonies from 1960--those are unsurpassable and can only be considered THE definitive performances of Tchaikovsky's last three symphonies). And the sound is fine--which is no small victory when it comes to Mravinsky live recordings, many of which were recorded by some of the most incompetent recording engineers known to mankind, a.k.a. Melodiya.
The surprises are the Beethoven symphonies. The performance of the 5th absolutely knocked my socks off--not only is it a GREAT performance, but I think it could possibly be a candidate for being the GREATEST performance that these ears have heard (my highest standards of comparison are Carlos Kleiber's VPO performance, Erich Kleiber's Concertgebouw performance, and a Furtwangler performance from EMI). The 7th is an excellent, vigorous performance. The first movement reminds me of Klemperer's approach to the symphony: grand, slow, monolithic. The 2nd movement is grave and beautiful, and the last two movements take off as they should. It's a fine performance. I haven't listened to the 6th just yet.
As for the Wagner...well, it's enjoyable because it's great music, and it's worth hearing, but no one will confuse these performances with being "traditional." There is definitely a "Russian character" to these performances (hard to describe in 10 words or less, but easy to recognize when you hear it) that sounds....well, foreign--at times I wondered if Tchaikovsky did some ghost-writing for Wagner. So because the Wagner selections are so different from what we're used to hearing, they are worth hearing. But more than once? Hmmm....I think I'll stick with Klemperer, Karajan, and Furtwangler. It reminds me of a concert I heard in which Tchaikovsky's 6th was played by the Boston Symphony Orchestra, conducted by Christoph von Dohnanyi. After the performance was over, I turned to my companion, and said, "Well, what did you think of that?" And he gave the perfect response: "That's why Germans shouldn't be conducting Russian music."