Neil. Nice report. I have done this exact trip twice this summer, once in early August(8/12) and repeated last Sunday (9/17). The first time was very fun and I would rate it in my top 2 or 3 favorite hikes in the east (I do not know if I like this or Huntington Ravine ascent of Mt Washington with Boote Spur descent better). Hikes involving King Ravine are in the mix too. The 3rd water fall is a bit of a challenge for me (I am no rock climber, I went with my friend who is a rock climber (can lead), he brought a rope as suggested in some guide books, I brought my harness, a caribeener and belay device. I have these from my glacier travel hikes out west). Other then that it was not too scary, but very fun scrambling when dry. It was also challenging getting out of the dike, we got out at 3700', maybe too soon ? Very steep but dry & grippy slabs. We did not use the rope.
Last Sunday was a different story. Although a sunny day and I thought a dry day on Saturday, trails around Avalanche Lake and the dike were quite wet and greasy. This made the climb much more scary then last time. This time I requested that my friend break out his rope on the 3rd waterfall and he also brought prussics which we used for safety to ascend the chimney to the right of the 3rd waterfall, using rope & prussic for safety(did not use belay device). I would have felt silly getting crippled from a fall with rope & harnesses in our pack, and wet rock made previously fun scambles quiet hairy. I also had a harder time getting out of the dike to on the steeper wet slabs. Slipped once, sliding about 10 feet and have some good rock burns on my arms & knees to show for it. I do not know why the dike was so wet, morning dew with no morning sun ? The east slide was dry and as you say the rock is grippy so this makes a great descent route (and ascent route in its own right, which I had done in the past). On that 3rd waterfall, someone had left a rope set up. I would never trust some elses rope. I do not know whose it was, left by rangers or well intended do gooder ? Or someone too tired to carry there 10 ton rope up the mountain after letting it dangle in the brook & become saturated ? The rope was set up to take right up the gut of the brook, no thanks !
The difficulty of the dike appears to very dramatically based on whether it is wet or dry, IMHO. I would reommend only going on a dry day with at least one or 2 dry days before it. I have read it is the sight of more rescues then any other route in the ADKs (I do not know how this book defines "route"). Pretty easy to see why. Non climber like me going half way up, it starts to rain, everything gets greased, can't go down, can't go up.
I did eagle slide last summer, consider to be like the upper reaches of the Colden east slide but the steep slabs go on for much longer. Another wonderful trip I want to repeat someday, also requires dry conditions to be safe or some rope for safety.
Cheers - John