They could have done worst than those last few songs. Like, put out a collaboration with Lou Reed or something... I wasn't so sure when I heard that first single last month, but after 2 or 3 listens, I could dig it.
One thing though - I find that this new bunch of songs sound extremely similar to their previous album, Death Magnetic. I felt that the type of production (minus the brickwall limiting) was appropriate for that album, what it represented and their return to their roots.
I do wish they'd moved away from that for those new songs, though. There really isn't much ambience - it's a very primitive type of sound, almost like a polished demo. It worked in the context of Death Magnetic - but that was 8 years ago. A bit more depth wouldn't hurt.
For what it's worth, they were my favorite band when I was a teenager - I was obsessed with Metallica. Unlike may early days fans though, I actually enjoyed seeing them evolve. As I started discovering things and moving away from metal, Metallica started experimenting and I could appreciate what they were doing. It allowed me to keep an eye on them.
I dug the Black Album, Load and ReLoad. The only thing that completely alienated me was St. Anger. I feel they had to purge themselves and that's what that one was.
Since Death Magnetic, I feel like they're celebrating their career. That album marked that final come back moment. From there on, they just need to stay on that track. They know what the fans want and they stick to that. I guess that's why they stuck with that sound. They're at peace with just being Metallica, just like Maiden has been exploiting the Maiden brand for over a decade. Nothing wrong with that.