I hiked the 12-mile round trip to Arch Rock, following the directions in the trail description. Actually, it came out to about 12.7 miles. As noted in the previous report, though, the elevation gain is wildly inaccurate -- I measured 1600 feet cumulative gain on my GPS. And the highest elevation, at Arch Rock, is about 5900 ft, not 5200 as stated in the guide. However, the grade is gentle throughout, so maybe it seems like it's only 500 feet?
A note on the road to the trailhead: FR 70 is about 17 miles long, and the last 6 miles or so are unpaved. For the most part it's in fairly good shape, but it does deteriorate the last mile or two. However, I had no particularly difficulty in my Prius. Also, although the main road is obvious at most intersections, I found the last one confusing. The road forks, with the left branch marked for Naches Trail and the right for Government Meadows Horse Camp. The right one is the one you want, and the trailhead is right at the camp. There is a latrine at the trailhead.
Like Oldwhiner, I was confused by Arch Rock itself. Having hiked just over 6 miles and passed the "Arch Spur" trail intersection -- which I assume is the one leading down to Echo Lake -- I figured I must be close to the summit. I continued on the PCT until it was clearly heading downhill again, then turned around and made my way back to the highest point on the trail. There is a large meadow just west of the trail, and using my GPS I navigated to what appeared to be the summit, at just over 5900 feet elevation. There was nothing resembling a rock or an arch, only a gently rounded summit consisting of spruce trees separated by small meadows. Nothing I would call a "prominent peak", as described in the guide. Was I in the wrong place? If not, does anyone know how Arch Rock got its improbable name?