I've got a Big Agnes "Lost Ranger" bag. Will the Large size fit this bag? Any thoughts on this pad versus the Big Agnes Air Core? Thanks!

Unfortunately it won't. The large NeoAir is 25" wide and Big Agnes' large size bag is still only 20" wide. Big Agnes makes their large pad 20" wide, but about 6 inches longer to fit inside their large bags. The NeoAir Regular would fit, which is what I would recommend, even though it isn't the full 6'6" length of the large Big Agnes Lost Ranger. By using a pillow (something worth carrying now based on the space and weight you will save by having a NeoAir) you can make up for the extra 6 inches. In terms of the differences between the Air Core and the NeoAir, the major difference is in comfort. Although they are both 2.5 inches thick, the NeoAir will be easier to find the 'sweet spot' or the point where you feel the most comfortable. This has been the advantage of foam based, self-inflatable pads vs. air only mattresses, but the NeoAir, with its horizontal baffles also does this very well. The NeoAir also offers R-Value, or insulation from the ground without sacrificing size and weight, unlike Big Agnes' insulated AirCore. The NeoAir packs down as small and weighs in at 14oz for the regular (19oz for the large). It is one of the hottest outdoor products of 2009 for good reason...