By and large, very good. The bass end is a bit out-of-control and 'boomy' on the cheap equipment I have hooked up to my computer, but that's OK. I've not been crazy about everything Horenstein has done because of his tendency to sometimes get kind of 'stiff', by sticking to a sort of 'one size fits all' approach to tempo relationships within a movement (his M4 and M7 are like that for me). But that doesn't happen here. The fifth requires a lot of gear changes from the conductor, and I think Horenstein makes those gear changes quite well. Of course, you can't judge an entire performance on one movement. But it's certainly a promising indicator.
I have only one negative comment: I don't care for the underplayed dynamics of the solo horn at the second spot in the movement - towards the end - where the solo horn continues on after the tutti horns have cut out. I prefer those dynamics be played as written. But that's not a deal breaker either. I can see why Horenstein buffs are excited about this. And if the other four movements are as good as this one, then all Mahler enthusiasts should get excited as well. It's certainly a lot better than the bizarrely conductor M5 I heard on the radio, just last night, with Tilson-Thomas/S.F. Symphony.