Yep it happend. After getting all the parts, some soldering, charging the Roomba, testing all the voltages (remember there are 14 to 18 VDC coming out of the Roomba, that would release the magic blue smoke super fast), connecting the ESP8266 Module, settings up the webREPL and firing the first commands nothing happened.
After connecting there the UART of the module to a UART-USB cable I saw that it actually echoes all the stuff I sent over webREPL. Damn it.
I will try it with using UART1 for TX and UART0 for RX, since UART1 is only unidirectional.