I was curious about this recording because I wanted to hear Maria Reining in the title role. While she disappointed me a bit (made some weird mistakes and ugly sounds here and there that surprised me--including in some of her big moments, like "aber der Richtige" and "und du wirst mein Gebieter sein"), I am very glad I bought the recording anyway. Lisa Della Casa proves that she was not only the best Arabella, but the best Zdenka as well! The high notes are no problem for her, and her vocal acting is superb. And Hans Hotter's Mandryka is the first really gruff sounding and "he-man" type one I have heard. Although lighter baritones often take the role, I am a fan of a big, craggy Wagnerian voice in this part, especially after hearing this recording. Many of the famous early Mandrykas were big voices like Kipnis and Schorr, and it definitely works well for the role.
As for the sound: it is probably better than you may worry it is...if you are a fan of live 50s mono, you will hear nothing unusually bad here. The words come across clearly, and the orchestral sound is only muddy in louder passages. I don't think it's an air check...sounds more like recorded in the theatre. Sounds about like Furtwangler's Der Freischutz from Salzburg. But you can hear samples in the Amazon mp3 store for this recording, so go listen.
I recommend this-just taking off a bit for Reining's performance...it must have been an off night for her.