UK user here. This appeared shortly after 7pm (GMT) and I've been listening to a selection of audiophile recordings through an Audiolab M-DAC into XTZ speakers. My immediate impression is that it sounds better than Audirvana with x2 upsampling enabled! (i.e. 44->88KHz only). However, I've often thought that upsampling benefits the atmosphere of songs sometimes at the expensive of dynamics, so I'll have to give Audirvana a chance without oversampling again to verify MC18's superiority.
Anyways... obviously there have been some glitches and crashes... but only when I was messing around too much, trying to find the View where all the album covers fly by at breakneck speed using the Graphics card acceleration. I guess that's a DirectX feature that is too difficult to be translated to Mac yet. (N.B. I haven't read any Release Notes!).
Nevertheless, if you just let it do it's job of playing music at High Quality from beginning to end, then there are no major problems and it gives us Audiophiles and FLAC-library-keepers a cause for celebration at last!.