I rode the trail from Ellensburg to just past the first tunnel and took a rest at the old cattle ranch homestead before returning home. It's a gentle 30 mile round trip and the temperatures were great, maybe 85. The trail is in really good shape, with 2 "lanes" clearly visible in the gravel. I was on a cyclocross bike and had Schwalbe Marathon 35c tires with no problems. The out trip was into the wind but the return trip was mostly downhill and I averaged 18mph-- that should give a good indicator of trail conditions to some.
Too bad the state could not spend less then 10 million to fix the tunnel and spend half of that on paving the trail. I think it would improve tourism in places that greatly need the extra income cyclists would bring. In Ellensburg, the trail gets heavy use from cyclists, runners and farm equipment (not legally from what I understand) and I expected to see a lot more evidence of horse use, but I saw very little in 15 miles.
Coeur d'Alene has the right idea and the rail to trails there are supposted to be toxic! The story there being that the rail bed was made out of heavy metal laden mine tailings; since most cyclists do not lick rocks or take trail side mud baths, I think they return ok.