I'm gonna be a bit of a contrarian on the Undermine music (shocker).
It's great, and I love myself some jazz every now and then. Heck, Jeeves and Wooster is one of my all time favourites.
I appreciate the team trying something different, and I appreciate the creative effort going into these tracks.
That said... for me, the Undermine music just goes waaaaay too far from the established WoW vibe. It almost breaks the fourth wall for me. It's as if Blizzard is speaking directly to me through the monitor, going "Haha, aren't these goblins fun and quirky Worldshaper? We love ourselves some random mobster action in WoW every now and then, don't we?

"
It's as if the Hearthstone team made their version of Undermine, not the WoW team.
I haven't tried the PTR so I'm sure I will be positively surprised when I log in and play. But from my comfy armchair developer perspective that I'm currently enjoying, I think this is comparable to those fantasy movies that play modern music in their trailers, with the "thun thun thun thun" beats to hype up the
action.
Imagine if LotR all of a sudden featured a completely different type of music than epic orchestral music, for The Two Towers. Like, opera or something. It would take you out of it instantly.
Okay, that's the end of my rant.

Yay, jazz. Nay, not in WoW.