For what it's worth, I did a lot of research into low-cost MCUs from the Chinese marketplace for my
$1 Microcontroller Review. I considered a lot of parts discussed in this thread. Before getting into the crazy 4-bit MCUs out there, anglophones may want to stick to the offerings from Holtek, Nuvoton, and STC. All three have good English documentation for their parts and wide availability on Taobao/AliExpress/etc.
Also, all three parts have decent development experiences (Nuvoton and STC have Keil; Holtek has HT-IDE3000), and their programming tools are low cost (STC can actually be programmed using a 3.3V / 5V TTL USB UART adapter).
Before ordering hundreds of dollars worth of stuff, dip your toes in the water with one of these three offerings and you'll find out if it's something worth pursuing further.
In my case, while I initially thought I'd be going back to these vendors more, I haven't found a killer application for any of these chips — and considering you can get fully-functional
SiLabs EFM8s for $0.32 each in single quantities, it's tough to justify the added development pain.
Sure, that particular EFM8 only has 2 kB of flash, but it has a much nicer peripheral selection than a lot of ultra-cheap MCUs: 18 I/O with 15 channels of 12-bit ADC, plus 4 timers, two comparators, SPI, I2C, UART, and PWM modules.
Food for thought! Keep us updated on your adventures!