Today I went on a 4 mile hike with the wife and kids. I was carrying my daughter in our baby pack(she weighs more than my 3 day pack) I didn't feel like wearing my hiking boots so I just wore my Nike Pegasus running shoes and really enjoyed a day without the weight of boots. This got me thinking about switching to just wearing running shoes on light trips.
Now the question is, what are the differences between standard running shoes and "trailrunners" (I have never owned a pair) I expect that the soles may be more rigid, but are there any more improvements to make them a better choice and make me fork over the cheddar.
The biggest differences I've noticed (in a good quality pair of trail runners) is on the soles. The grip is usually a bit more rigid, and often has a very lightweight stiffener. I find this helps for going up steep inclines, and also helps you not feel every ridge and sharp rock. The stitching is usually a bit more solid, and overall the shoe is more rugged. This said, the trail runner shouldn't be much heavier than a good running shoe.
I'm currently testing out a pair of Montrail Continental Divides (28 oz/pair, Men's 10.5) and so far they're great, and no more expensive than quality running shoes. Montrail seems to 'get' the trail shoe, plus they have environmentally sound business practices (their shoes come with no extra packaging, stuffing, marketing, or labels, just a box.)
The only differences I've noticed are better treads and a tougher shell on the trail runners. I own a pair made by New Balance, and they've held up very well for about 6 years of use on trails and concrete. Mostly concrete.
Trail runners are fine for just that - trail use, with running as an option for the gung-ho among us. I wore a pair (lined with gore-tex) on a six night offtrail hike thinking about how my feet would thank me, and those shoes pretty much fell apart by the last day. They even lost a big hunk out of the heel. They're still great for what they're made for, though.
A few years ago I switched from boots to Solomon trail runners. They are holding up very well. Better than any boots I have owned to date. The soles are very tough. They keep my feet nice and dry and are very comfortable. I love the lacing system too. Just pull on the toggle to tighten or loosen to suit the need. For me they are the way to go. I was actually a skeptic before I tried a pair. I bought my sister a pair and she loves them too.