I learned some new manufacturers that I didn't know from this thread and then I got curious to try some of their chips.
So for what it's worth here is my experience.
I got a Cortex-M3 chip from MindMotion (MM32F3270), a Cortex-M4 from Artery(AT32F413) and and another from XHSC(HC32F460).
Mindmotion: First of all the website is in English and it is reasonably easy to navigate. They provide a decently written documentation (User manual, Datasheet, Errand). They also provide a Keil package and a zip with their HAL and plenty of examples.
So getting your hands dirty quickly was surprisingly straight forward for me. The code in the samples was very clean and easy to read and I also liked their HAL APIs as well.
In the negative side I found it annoying that their Keil package only includes SVD files for register descriptions. So I had to convert the SVD file to something that Keil could understand so that I can get peripheral register explanations.
Artery: Again the website is in English and is easy to navigate. The documentation of this chip really impressed me I have to say. The provide all the usual documentation in English (User manual, Datasheet, Errand) and additionally they provide getting started guides, migration from STM32 chips, product selection guides and more. The quality of the documentation is also quite high, I had no problem to navigate and find the information that I wanted. In the IDE side they provide a Keil package, and also their own Eclipse based IDE which I ended up trying. (They also have a GUI configuration tool for the pins configuration of the chip, similar to what the STM32 has). Their Eclipse based IDE worked without issues to program and debug the chip. Their HAL code quality was pretty good as well with clean and easy to read APIs and HAL functions and they provide plenty of samples as well. The only slight negative that I found was that, even though they provide plenty of examples, they don't support opening their examples projects with their own IDE.
XHSC: Their website is trolling us, they have an English button in the website which does nothing
I eventually realized that they have a reference manual document in English in their website but that's that only English document that I could find. Searching the internet revealed an English datasheet as well. Both of the documents were not written in good English and it was not always straightforward to understand what they try to say. They also provide a Keil package and examples as well. Getting a blinky to run was pretty straightforward. But it was quite time consuming to go further than this. Their HAL functions were not very straightforward and and overall code quality was not as good as the other ones I tried. To be fair though, the chip itself seems pretty good feature and it is probably the most feature rich one, so maybe that played some role.
In the tests that I did I build an example with a simple use case:
1) Sleep until there is an interrupt
2) Receive 10 bytes from UART and copy them from UART -> memory through DMA
3) When the 10th byte is received wake up the chip and blink a LED
I would really be interested to hear if there are other opinions here