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:

I think this could be real competitor for Arduino. It has a ARM core which is much better than the AVR core of the arduino. And the price is comparable too.

Yes, I think their offer is very good and it covers a wider range. The Nucleo 144 are real beasts with no Arduino equivalent.

Yes dear..
@parceval

Is the code Arduino compatible? The advantage of Arduino is also that there are so many open source libs and examples.

If you use the stm32duino there are standard libraries that are already ported.
You can check those threads for more details :

If you want to put shields it is another story I think, voltage and features on a given IO can be problematic. It depends on the target board you want to use.

You got a 1.53% upvote from @postpromoter courtesy of @boucaron! Want to promote your posts too? Check out the Steem Bot Tracker website for more info. If you would like to support development of @postpromoter and the bot tracker please vote for @yabapmatt for witness!