While Davis' RCA recording of Mahler 8 is flawed for reasons I'll delve into in a moment, it's a steal at the used prices that it's going for (you have to pay the shipping too, of course).
Flaws: the vocal soloists are sometimes too close, especially during Part I, and the during the 'three penitent women' passage towards the end of Part II (the soprano and mezzo solos). Also, the organ is a tad underpowered.
Strengths: the choral work is outstanding. In fact, I think this performance from Munich's Gasteig hall might have had some electronic help for the soloists and chorus. Maybe. In addition, Davis sounds completely at home in such a huge work. It's as though all his prior experiences in Berlioz pay big dividends here. He never sounds embarrassed by the Mahler's work, or as though he's trying to micromanage it.
What really puts this over the top, for me, is the way Davis handles the end of the symphony. He employs a slow tempo at the spot where the chorus sings their very last syllable - the spot where the trombones reiterate the "Veni Creator Spiritus" theme. Thus, what he hear - if I can try to approximate it on paper - is "Hiiiiiiii-Naaaaaaaaaaaaaaaaasaaaaaan". In other words, the chorus really hangs on to that last syllable, and at a full fortissimo. It's positively thrilling! Also, the series of tam-tam smashes (large orchestral gong) that follow sound huge here. Much bigger than on most other recordings.
So yes, this is a flawed recording, but one that truly builds to a great culmination (and yes, the organ could be a tad stronger too). Also, tenor Ben Heppner turns in a fine performance on his two big solos in Part II. In fact, he sounds a bit better here than he did later on, which was on the Chaily/Concertgebouw recording (Decca - and certainly better sung in terms of the other soloists). At the very worst, one wouldn't waste much money experimenting with the Davis. I won't part with mine.