Weighing in at a feather under 1,600g for the pair, these Fulcrum Red Metal XRP 650B hoops are crazy light. They're also tubeless-ready, and are intended for anything from cross-country racing to all-mountain hammering. At a penny under a grand, they're really not cheap though.
The hubs' part-carbon bodies and the extensively machined bodies and rims must go a long way to accounting for the low weight and high price of these wheels.
Their scant weight is noticeable instantly – this middle sized wheel accelerates incredibly fast. They're very stiff too, and sometimes seem like they might deflect and ping off liner ather than track the ground on rough bits of trail.
The rims have stood up well to a repeated battering, although the wheels have needed truing. Our biggest gripe is with the bearings – they are a fiddle to adjust and have needed constant maintenance and a full rebuild just to keep them smooth. The front bearings have been less reliable than the rear.
If you're willing to put up with high maintenance in return for light weight though, these could be worth a shot.