Hi TOMAHAWK70. I recently tried out trail runners on an 11-day, 165-mile hike of a relatively unrocky section of the AT. Prior to this I have always worn a pair of Danner 8" boots, which performed rather admirably in my opinion. The only reason I switched, or considered switching, was a single incident where the boots became waterlogged in extremely heavy rain. My socks had gotten soaked and started to pull water into the boot, and very shortly it felt like I had tied dumbbells to my feet.
So this trip around I went with a pair of semi-low-cut
Montrail Storms. They're not the lowest-cut trail runner; there's about an extra inch of material that covers the ankle. However, this kind of shoe still lacks the ankle support of a taller boot, so on the first few days I kept twisting my ankle on rocks. It was nothing serious though, since I had my hiking poles to balance my weight and to walk off the pain.
During the middle of the hike, I realized that I was twisting my ankle less often. I had started to adopt an unconventional shuffling gait that used locked ankles (less wear and tear on the muscles, especially the anterior thingamajig, letting me walk further each day without pain). I really had to lean more on my poles this way, but I suppose I was just distributing the work to the upper body which could handle it. I didn't really experience much of the "ankle-hardening" that extreme long-distance hikers talk about-- I've heard that the ankles become much stronger after the first month or so of continuous hiking. So nope, I was still near-crippled for a few days after my hike, with one ankle almost unable to flex without an incredible pain in the anterior muscle. I suppose this pain might be some kind of "shin splint", but I hadn't experienced anything so intense when I wore combat boots.
In spite of the weakness in ankle-support, trail runners do have some things going for them. They dry out more easily, so on the drier nights they have at least some chance of drying out. My pair had a decent resistance to water, even when I was standing in 2-3 inches of it, but the fact is that this kind of shoe is eventually going to get wet. It's just that there's a chance that it will dry out someday, and the extra water in there isn't absolute hell to lug around.
Also, trail runners are easier to take off during short breaks, if you do that sort of thing. I didn't take mine off much though, mostly because I try to keep a "just keep going" mindset. It's also much easier to slip blistered feet into them at camp. Having trail runners, I wouldn't even consider having those lightweight rubber camp clogs (they're dead weight, if you ask me).
I did not notice much of a difference in temperature between wearing trail runners and boots. When you're out there, your feet are always cold (and clammy, and stinky), but the pain of the mileage is always a greater concern.
I didn't weigh my pack, but I estimate it to have been about 35-40 pounds most of the time. With this kind of lighter-weight load, it's reasonable to wear trail runners, but I'd do that only if hiking poles are also being used. I can't imagine how painful it'd be without them. All in all, I'm split pretty much even between boots and trail runners. Hope this helps.
