Was looking for a first day out on "real" trails with my 9-year old. This park turned out to be perfect for what we were looking for, my daughter especially enjoyed her first singletrack on Ravine Run, Salal Trail and the section of Spine Line between the end of Ravine Run and Miller Bay Road (looks like some recent MTB-specific improvements on the switchbacks). There was a bit of pushing involved to get over the long climb heading west on Spine Line, YMMV ;-)
Conditions were generally good, trails damp with only a few muddy spots. Some trail repair work is underway on Boundary Trail, only a short section. There were a few trees down from the windstorm last week, nothing you couldn't easily lift your bike over.
An adult rider could probably ride every trail in the park in about an hour; we made a whole day out of it by parking on the Edmonds side of the water, riding onto the ferry, then riding on the road from the Kingston ferry dock to the Norman Road entrance to NKHP. (1 mile of busy road with a designated bike lane, then 0.5 miles on Norman Road which only serves about 10 houses and has zero traffic).