Well first of all I want to apologize if any of my comments about limited support seem disparaging to those on this forum; this was not my intent.
I have desperately wanted to use the STM32F4 and now L4 for at least two years but have been stymied by the difficulty of learning how to use the simplest non-Arduino toolchains including attempts with Keil, Crossworks, Eclipse and mbed. I got furthest with mbed and was able to translate a few of my Teensiduino sketches and make use of the STM32F401 Nucleo board to read sensors and do simple sensor fusion.
I started designing a Teensy-like STM32L4 with the intent of using it in the same way but was happily surprised to receive an unsolicited offer to help from Thomas Roell, who has been working to write from existing code and scratch an Arduino overlay that will allow the STM32L4 (and F4 later) to be programmed over the USB cable with an Arduino IDE just like the teensy. The hardware design took quite a bit longer than I expected as is the software, but the hardware is essentially done. I am waiting on the final proof pcbs from OSH Park and then I will order a small build from China for additional testing and the start of regular sales at Tindie, where a few prototypes are listed mostly for advertisement.
The Arduino IDE is intended to support most if not all of the usual serial (SPI, I2C, UART, etc) libraries as well as a drag and drop file system for the use of the on-board 16 Mbyte QSPI flash.
I expect to use the Dragonfly for portable (wearable) applications and remote data logging. I have designed the board with large solder pads on the back so I can solder small add-on boards holding sensors or radios, etc. I only have one so far with the MPU9250 and BME280 which together with the 16 MByte QSPI flash and the low-power, battery operated nature of the Dragonfly will make a great remote environmental data logger.
I have programmed the Dragonflys I have assembled with dfu-util using precompiled binaries provided by Thomas. I am anxiously awaiting delivery of the first basic Arduino capability, which i expect at the end of April or so. Progress is slow since there is essentially only one person doing all of the detailed work to make sure USB works, etc. My hope is that after the basic capability is developed and made available, users will step up and make the device as useful as it can be. The latest design of the board is here: https://www.oshpark.com/shared_projects/xTPoZ2ID
and we plan to make the software open source as well.
I would be interested in any comments and/or suggestions for improvement. It's a very exciting project for me and I am looking forward to using the Dragonfly for all of my work.