These are light but enduro/DH tough. They're nice & wide as required by some of the new wider trail tyres and they'll give a bit more volume to standard tyres.
The ZTR range of rims has made Stans NoTubes plenty of people's go to brand. For wheel builds people ask me for Stans 90% of the time. They do seem stiffer than the previous version which was already one of the best MTB rims.
The Mk3 range has been updated with a stronger alloy and a better profile to allow them to be wider, stiffer & stronger whilst losing a bit of weight.
They're a trusted brand & although one of the innovators they've also been around for long enough to know how to make reliable products.

The price is for one rim, but a pair is available. If you want a top quality build, just buy & get in touch for a price. I can source hubs or build onto your current ones with Sapim double butted spokes (the best IMO). Black DB spokes & build come to an extra £48 for 1 wheel or £91 for a pair, plus the cost of hubs.

NoTubes' description:
The new Flow MK3 is wider, stronger, and lighter. The Flow’s legendary history on the World Cup DH circuit, combined with our WideRight 29mm internal width offers the perfect balance of tire stability for outstanding control at any speed. Genuine Bead Socket Technology rolls faster, corners harder, and withstands even bigger hits. Combined with a new shape, the Flow MK3 is born and bred for control at speed and perfect for technical trails and enduro racing. Laterally stiffer and more durable, our new alloy is also even lighter, putting the new Flow MK3 in a class by itself for all-mountain performance.

Rim Internal Width: 29.0mm
Rim External Width: 32.3mm
Rim Depth: 16.0mm
Hole Count: 32-hole
Drilling Style: Symmetrical
Color: Black
Material: 6069 Aluminum Alloy
Brakes: Disc only
Rim Weight: 480g
ERD: 567mm 
ETRTO: 584x29.0
ISO: 27.5"x32.3mm
Tape: 30mm
Valve: 35mm Presta
Spoke Tension: 125kgf (1225N)
Max Pressure with 2.35" Tire: 37psi
Max Pressure with 2.8" Tire: 26psi