After using Keen Targhee 2's for several years, I decided to try the ever popular Altra trail runners this past year on my section hike of the Shenandoah Nat'l Park.
Why? Because the waterproof hiking boots will eventually get saturated - even hiking through dewy grass - and they take two consecutive dry days to fully dry out. Lighter and more breatheable trail runners will dry faster and the energy saved at the end of a day of hiking is said to be significant.
So I went to the national chain box store outfitter and tried on every pair of trail runners they had to see which ones fit me the best. (I did this previously with boots, and taking the time to do so is worth it; I got my first blister in my Keen's after 500 trail miles, doing a 19 mile day in the rain.) I settled on the Altra Superior 4.5, and installed the supplied rock guard between the sole and the insole.
*edit* The first thing I noticed was that the flared out sole of the right shoe kept hitting my left ankle as I walked. Annoying, but after a couple days I seem to have adjusted my gait to stop this.
The middle part of the SNP is pretty rocky - and it was my longest day of the trip at 22 miles - and my feet were killing me from that point onward. I also kicked a rock or root, and ended up losing a toenail.
I did not get any blisters on the 112 mile section.
I have come to the conclusion that my feet need the added protection of the heavier boots. Perhaps if I ever get a chance to do a thru hike, after a month or two of hiking with boots, I will switch to the Altras again, hoping my feet will be conditioned to hiking by that point. Just doing one (or two if I'm lucky) section a year, however, my feet don't have time to get toughened up enough to use trail runners without suffering for the next two months.