We fully enjoyed experiencing the Cape Horn Trail and can understand it's appeal. The trailhead parking was overflowing when we arrived at 10:30am. The camaraderie was palpable though, in the line at the toilet and along the trail. Everyone seemed tolerant, upbeat, and friendly. Probably because it was Saturday, the sun was out, and a "Re-Pete" was still plausible.
We were a bit apprehensive about the detailed route descriptions on this site and the Portland Hiker's Guide. However, we found plenty of signs, well-used paths, and no need to look for obscure directional clues. Common sense and the kindness of strangers resolved any confusion. Words of advice: 1) Don't bother following a "horse trail" sign unless you are actually on a horse; 2) All trails marked "viewpoint" still lead back to the main trail.
The loop trail is bisected by the highway. We took the loop counter=clockwise with the first half of the hike "above" the highway. Enjoyable views and wide portions of trail in the upper half. After crossing through a pedestrian tunnel under the highway, we began the lower portion of the trail. The route became much more sketchy, narrow, and rocky, but most enjoyable for the nearness to the Columbia and the waterfall. In retrospect, we wished we'd taken our lunch break before we hit the lower trail though, as its narrowness and exposure to extreme winds precluded a leisurely stop.
The lower trail spits out onto a paved side road a mile or so below the main highway and trailhead parking. No traffic on this road during our walk back to the car. Just a pleasant stroll past farmhouses and pastures, through another pedestrian tunnel, and to the still=full lot.
Well worth it - even with a crowd. We'll never forget the views or the roaring wind which sounded like surf and tore our caps off!