A tale of 2 trails... The bottom mile is a pretty dull gravel road that is a city of Dublin trail and by itself I would rate 1 1/2...
You then go through a gate to the Contra Costa part of the trail and it is very good at 4+.
The good thing about both sections is that it is mostly shaded. I also notice a couple of single tracks that if I ever did it again I would explore. The trail goes beyond what is mapped here as well...