Have had great success with Easton wheels (EA90SLX) on my road race bike (in terms of weight, durability and quality) So Easton were always high on my fave brands list. In the market for some new wheels for my MTB for marathons and stuff. Are the XC ones worth a look, there are some great deals about, but also some mixed reviews, main concerns are the availability of spares (particulalry spokes, as they use a threaded both ends type I believe). And overall durability. Be intereseted in hearing from people who have used the XC Ones. WMB review liked them, but that was a short term test.
I'd personally hand build some according to your criteria, the XC Ones aren't that bad considering their weight, but the availability of spares thing is always a problem, I'll wager you could hand build some lighter (or tougher) wheels for less.
[b]njee[/b] is right about spares; the XCOnes have 24 straight pull spokes per wheel so you'll be stuck with Easton parts. Whethr or not this is a problem will largely depend on how available Easton make those parts and at what price.
As for the wheels themselves, Merlin have had them on at good deals with brakes for a year or so now and I have 2 pairs; one set on my Tranny and one on Mrs Fascist's Rocky Mountain Blizzard. They're a lovely stiff wheel, much stiffer than other factory lightweights I've tried. I assume this comes from the deep section rim. The freehub is nice and quiet, engages as well as any mainstream freehub (ie, not instant on like some i9, CK etc). In short they're a very competent wheelset for XC, very raceable although I'm not sure how far you can push them, I've ridden them SS in stuff like the North downs without a problem and I'm not too careful with my kit.
nice wheels although I expect that njee is right about a handmade set. not sure how much a set of XT on 719s with DB spokes would weigh, but they'd be cheaper than XCOnes unless you found a very good deal
For me, I like 'em, I just wish they were tubeless out of the box, as adding stan's strips somewhat negates the weight gains that you're paying $$ for.