The new Raspberry Pi 3 released on 29 Feb 2016 has issues with its UART port as the pinout GPIO 14/15 on the pin header is now based on a low throughput mini-UART.
To understand the issue better than reading the wall of text below, you can see the talk I gave on this issue.
This mini-UART does not produce a stable baud rate as it fluctuates based on the Core clock speed whenever it rises or falls. The result is that the serial debug output is practically unusable. On Raspbian you may see garbage or nothing at all. The solution is simple, add
/boot/config.txt to cap the core frequency to a constant value. This optio