On my 1520, I'm seeing this build as a bit of a downgrade over 10572. Although there have been minor improvements to performance and battery life, it has come at the significant expense of stability and functionality.
The SD card issues remind me of the similar bug back in build 10166, it shouldn't have reared itself again so much further into development. Additionally, I've had my phone crash and reset itself twice now doing different things, once when replying to a text and once when opening a new tab in Edge. In addition, after about two days of running, the phone slowed to a crawl, with apps constantly crashing, requiring a soft reset to restore performance.
This is in stark contrast to 10572, which whilst not quite as fast as this build, but was the first build I've used that had every piece of functionality working. Zero crashes, no need to soft reset at any time, and (finally) good battery life compared to previous builds.
Overall I'm a bit apathetic towards this build. I hope they didn't skimp on QA for this build just so they could push it the same day as a desktop build. I'd have gladly waited another few days (considering how long we've been waiting in the past between builds) for one that was more stable and had fewer problems.