It promises to maybe be faster. Has no internal flash, cannot be bought in volume if you intend to actually make anything, has a very immature toolchain, and currently is a theory only.
These SiFive folks claim to have working silicon, there are what look like some power measurements on their site, and there are photographs of boards available for a claimed ship date in ~3 weeks.
It's not in my hands personally, but they seem to be over the vapor hump.
I found it an exciting enough product to drop $60 on, anyway.
Your disclaimers are fair in that if you want an Arduino, buy an Arduino. But if a 320 MHz anything is slower than a 24 MHz AVR then the sky is falling.
