In reply to gavinpeach:
I have Western Mountaineering Versalite:
http://www.westernmountaineering.com/index.cfm?section=products&page=Sl...
Wife has PHD Hispar 500 but it was with "800 down" before they discovered the "900 down" and the tooth fairy.
http://www.phdesigns.co.uk/product_info.php?cat=26&products_id=245
The PHD weighs slightly more although the WM Versalite is the Long version.
Both about the same warmth, Western Mountaineering better made, the PHD zips are awful, jamb because they dont put hard faced band next to zip to stop material catching. Save weight (rubbish since WM do sew one in, save them a bit of cost)
Used the WM for camps at up to about 6500m on snow and ice.
Worth considering a DownMat 7 since you loose a load of heat downwards.
A lot more robust than you would imagine and not that much hassle to inflate. A lot warmer and more confortable than same weight of Thermarest.