I was having a quick look on the Mountain Equipment website and noticed that the Xero 150 sleeping bag isn't listed anymore. I had looked at the Xero 150 as I already use a Xero 250 which I'm very happy with as the elasticated section reduces the size of the bag without feeling restrictive. There is mention of a new bag, the Xero MM (I assume that implies Mountain Marathon) although as yet there are no images of the bag. The interesting thing is that they mention a product known as the 'Balloon Bed' saying that it helped inspire the new Xero MM.
To be honest although I'd read about people making air mattresses using balloons I wasn't aware that anyone was actually selling them. It appears that there is, http://www.balloonbed.com/ and while it isn't something that really interests me I'm curious to see what the Xero MM has to offer.
With a stated weight of 400g with 175g of 750 FP down it's closest competition would probably be the PHD Minim Ultra Down with as far as I know 150g of 900 FP down. Reading between the lines it appears that the Xero MM may incorporate channels on the base to allow some kind of air chamber to be inserted, whether those chambers are re-useable and supplied with the bag or whether like the 'Balloon Bed' you simply use modeling balloons remains to be seen but it's certainly something different.