From the GitHub release page:

Woohoo! After our first stable release with 1.0.0 we’re back to breaking things while we improve them. DO NOT use this release if all you want to do is try CircuitPython. 1.0.0 is much better for playing around.

However, 2.0.0 is shaping up to have some awesome new features. Here is a short list:

• Update to MicroPython 1.9.1 which supports mounting multiple file systems at once.
• Introduce a storage module to facilitate mounting and unmounting file systems.
• Replace uos with os which is a subset of CPython’s os
• Replace urandom with random which is a subset of CPython’s random
• Increase heap size from 16k to 20k. This means more code can be loaded. Thanks @dhalbert!
• atmel-samd: Introduce audiobusio.PDMIn for recording sound to memory from a PDM microphone such as the one on the CircuitPlayground Express.

Moving from 1.0.0 to 2.0.0 is not a perfect process. There are a few things that break when doing so.

• mpy files compatible with 1.0.0 are NOT compatible with 2.0.0 because MicroPython changed the format. A beta bundle is available here. (Its built by changing the branch of circuitpython in the bundle repo before building it.)
• uos is replaced by os, storage and multiterminal
• urandom is replaced by random

This release is based on MicroPython 1.9.1. Support upstream MicroPython by purchasing a PyBoard (from Adafruit here).

Exhaustive change list since 1.0.0.

Have a Feather M0 Express, Metro M0 Express, Gemma M0 or Circuit Playground Express? Download a .uf2 and follow these instructions. To install our libraries simply download this beta bundle and copy the lib folder onto your CIRCUITPY drive.

To install on legacy boards, download a .bin file and follow the guide here for M0 and here for ESP8266.