it's open source all the way down to the micro-controller's HDL.
Got mine yesterday, haven't gotten further than reading the documentation. But it's only the processor core that's open source, not the whole chip.
The entire SoC is open source.
The HiFive1 board contains non-open components. An example is the 128 Mb flash memory, though that is fairly trivial and doesn't contain a lot of IP. The more notorious one is the FTDI USB chip. I think there simply wasn't another good open option at the moment, especially as they wanted to support JTAG via USB for debugging and for programming the flash.
Even RMS started the FSF with just emacs, bison, ld and gdb usable. (gcc came soon after)
My Founder's Edition HiFIve1 finally arrived in Moscow on Friday and I managed to wrestle it quickly from the hands of Russia Post despite the language barrier :-) Sadly all my Arduino accessories are in NZ, so I'll have to find some more here.