Several World Cup XC racers use the ZTR rim. At 350g, it's one of the lightest rims out there and it's tough too.
The ZTR design eliminates the bead hook of other rims and uses Stan's bead socket technology - the rim's sidewall curve is the same shape as the tyre bead so they fit like a ball-and-socket joint. This drops the rim wall by 2-3mm, losing some weight, but then some has been added back to boost strength and stiffness in the rim centre. The result is less strain on the tyre bead and a tighter seal.
In theory, the short sidewalls mean the tyre shouldn't hit the rim and pinch flat as often as standard rims, which is great for those who like running low pressures. So far, so good - no flats, and the rims are still running true after 500km or so.
This could be the best XC racer's set-up but don't ignore it for other uses. Low spinning weight plus lots of strength and stiffness is a big asset on any bike. The only reason this hasn't got a 10 is an imperfect join on one rim.