I really like this shoe.  I got it to replace a pair of New Balance 790's for trail racing.  The 790's were good, and I won some races in them, but always felt a little too skittish in the traction department, especially in wet conditions.  When the 840's were announced, I was interested right away.

I've worn New Balance racing and running shoes for the past 17 years, and I was confident enough to buy this shoe online without trying it on.  It does feel a little bit different around the ankle compared to my recent NB shoes (more padding around the ankle cuff, mainly - which is not a bad thing), but the fit gave me no problems and I actually like it.

Compared to the NB 790's, the weight is very similar (i.e. verrrry light, suitable for racing), but the traction is much better.  This seems to be due to both a more aggressive tread, as well as softer rubber.  I bet the rubber soles won't last as long on these new shoes if you spend a lot of time running on pavement with them, but that's not what they were designed for anyway....

Mud clearance is great, even in tough conditions.

To keep the weight down the uppers are made in a minimalistic way, so durability is not likely to be a high point with these shoes.  However, if you want FAST, LIGHT SHOES SUITABLE FOR TRAIL RACING, and the New Balance last fits you well, these shoes are for you.

I will still race in my 790's in dry, less-technical conditions, but when it gets wet or if the surface looks to be more technical, then these will be my go-to shoes.