Hi !
The STM32 Nucleo plaftorm rocks, it is way better bang for the bucks than Arduino for DIY, especially if you need a lot of IOs.

The offer is very complete and it offers interesting key features:

• Wide range of 32 bit ARM Cores: ARM M0, M3, M4 and M7 !
• High speed DMA controlled 12bits ADC (up to 2 Million Samples per second).
• Correct DAC on 12bits for some of them with I2S
• Integrated Debugger/JTAG/Programmer: for debugging it is really important, Arduino are lacking this.
• 5V tolerant IOs: robustness, less easy to fry board.
• Different number of IOs: 25, 50 to 144 !
• ARM MBED Libraries and various supports in RealTime OSs. It exists an Arduino IDE port with libraries, but I do not use it.
• Very competitive pricing especially on the 50 and 144 boards: 10, 15 to 24 USD
• Ethernet port on the 144 boards
• Widely available through high reputation vendors: Mouser, Farnell ...

Follows a few links to give you an idea of the platform:

• 25 IOs : https://os.mbed.com/platforms/ST-Nucleo-F031K6/
• 50 IOs: https://os.mbed.com/platforms/ST-Nucleo-F411RE/, https://os.mbed.com/platforms/ST-Nucleo-F446RE/
• 144 IOs: https://os.mbed.com/platforms/ST-Nucleo-F767ZI/, there is a new board that goes up to 400 Mhz it allows to do very interesting things for a 24 USD board: NUCLEO-H743ZI

Personally I own a few NUCLEO-F411RE, NUCLEO-F446RE used for hobby and prototyping. I converted also a friend who is an Arduino addict to this platform.
I use a Cross GCC, ARM MBED, OpenOCD and StLink.

Links:

• https://github.com/ARMmbed/mbed-os
• http://www.openocd.net/
• https://github.com/texane/stlink
• http://www.stm32duino.com/