We did this hike in July, 2005. It was stifling hot in Springdale, but the combination of the cool (but not uncomfortable) water and the high canyon walls (which keep you out of the direct sunlight) makes this hike the best choice during the very hot days of summer.
We're a family of four - parents in our mid-forties, the kids were 12 and 10 last summer. We rented river shoes/socks and walking sticks ($16/person) at an outfitters in Springdale and received a brief orientation talk about the Narrows, something I would highly recommend.
We started at about 11am, after packing lunch, cameras, wallets, etc. in ziplock bags to keep them dry in our packs. I would highly recommend quick-dry clothing, like running gear or soccer jerseys; definitely no cotton. The walk from The Temple of Shinawava (sp?) to the end of the Riverside Walk was very comfortable in the river shoes, and soon we were in the river. There are always crowds of people at the trailhead, as people wade around in their sandals. As soon as you go 500m or so up the river, however, the crowds thin out, and only those committed and properly equipped groups are left.
After you get used to the feeling of the current on your legs, and how to use the walking stick to get the best balance and leverage, it was a piece of cake. Even the 10-year old had no problems whatsoever. The views of the canyon walls are truly breathtaking. It's unlike anything we've ever done before. I'm sure there are canyons like this elsewhere, but the combination of easy accessibility, safety (there were no threats of flashfloods the day we hiked), and dramatic canyon views are probably unique to Zion. We had lunch at a nice shady spot at a bend in the river, right near a big rock, which overhung the river, and we could see small fish swimming in the calm, shady water under the boulder.
As we kept going, the walls of the canyon go continue to narrow, and by the time we reached the area called "Wall Street", the effect was truly amazing. There's a good picture on the Zion NP website, but even that really doesn't do it justice (it's like trying to take a good picture of the Grand Canyon). As we made our turnaround to go back down river, it was almost a bit sad, that we wouldn't be able to see more of the upper canyon. But that was soon forgotten, when we realized how much fun it was to go downriver. There are spots where the river is deep enough that you just put your feet up and drift with the current. My kids were squealing with delight!
By the time we got back to the Riverside Walk trail, we were both tired and exhilarated at what we'd seen. It was truly one of the most memorable times during our 2-week trip from Vancouver to Arizona & back.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.