Purchased on the recommendation of the sales person at Joe's Sports in Salem, OR., and I am very impressed with this shoe. Walking in the store on a hard surface, it felt like the heel's outer was pushing my foot inward (I have a so-called "regular" foot, no pronation or supination) and indeed, it does cause my foot to roll a bit inward.
But running/jogging is another story completely. On a bark dust path, this shoe is absolutely the best I've ever worn. Great grip. Great stability. On hard, paved paths, it works well, but I still feel that inside push at the heel. Some running shoes can be worn all day on the job, but the 840 for me is a running only shoe.
I have not used the shoe on serious trail running with ruts, stones, etc., but after 30 or so miles on bark dust and grass, I am sold on the shoe. Breathability is a 5, traction on grass and mud is a 5, fit/comfort is a 5, lacing is a 5, lightness is a 5. If style/color were significant to me, I'd give it a 1 because the orange/yellow/gray scheme is truly butt-ugly, but I don't care, so no score on that one.
Retail was $85, I got it on sale for $60, and will buy another pair tomorrow to keep as a spare.
Fit issue: I wear a 9.5 or 10 in every other brand...had to buy 10.5 in the 840.

I am a 64 year old runner who was looking for a lightweight shoe to do fast paced running on grass and dirt trails. I run slower workouts in the Asics 2400 trail shoe but it is fairly heavy. The New Balance shoe seems to be exactly what I was looking for

The New Balance 840 Trail Running Shoe may not look tough, but as Mark Twain once said, "It's not the size of the dog in the fight, but the size of the fight in the dog!" The NB 840 has more fight in it than another other trail shoe I have ever owned.

First off the NB 840's light upper makes this shoe incredibly ventilated, all but eliminating heat friction and discomfort in hot environments (like many other trail shoes with liners and thick uppers). The light upper does let water in very quickly -- a possible down side -- however, due to the shoes overall "slipper" like low-silhouette construction, and excellent ventilation, water seems to be drawn out of the shoe just as fast as it arrives.

Next is its durability. I mountain bike on flat peddles, adventure race like there's no tomorrow, and run my brains out (on trail and on pavement). The NB 840's highly flexible construction allows it to take a massive amount of hammering without having its various parts peel away or split open. After three 12-hour adventure races, six mountain bike races, and over 100+ miles on pavement and trail, the only points of failure these shoes have is in the stitching along the outside of the toe and snag holes (from branches and rocks and such) in the uppers meshing. A majority of the tread still remains as do my original laces that came with the shoe when purchased 9-months ago.

Finally is comfort. As I mentioned before, this shoe is like a slipper. It was easy to tie a square knot into the laces and then just slip the shoe on and off when needed. The low-silhouette does come with its limitations -- primarily ankle and arch support. The NB 840 makes you feel like you are running the trails almost barefoot. However, pavement can put a hurt on your arch -- which is why they make Super Feet insoles -- and there is little to stop a major ankle roll. But with the shoe so low to the ground, I found that my ankle rolling days where behind me with this shoe since I had superior control of my footing -- both due to claw-like tread and better use of my toes.

I might say the only down side to this shoe is its uncanny ability to find a tiny rocks to drop in while off-trail running (especially bushwhacking during orienteering). But I found that thicker socks -- or even gaiters -- causes that little annoyance to go away!

Overall, this is THE best trail shoe I have ever owned. It can take anything that I can throw at it I and still function well beyond my needs. And at a price that pays for itself simply due to it's capability to outlast the competition.

I only wish they would make the shoe in more colors, but for now, racing stripe yellow will have to do for a shoe that has one hell of a fight in it! Nuff said!

They are light and bouncy on a flat surface but...if you're running on any sharp rocks or tree roots, you'll feel it! Running on grass or a well maintained trail are where these shoes really shine.

These shoes are legit. The traction is amazing. They are so light and they are extremely comfortable. Awesome shoes!