Can't beat down for warmth.
This was an upgrade from the previous Rab Ionosphere 5.5, which wasn't warm enough. This has an interesting construction with internal foam blocks head and tail, presumably to keep the down in place. That makes it slightly bulkier. No big deal. Tested just once so far (I've only had it a week) on a 5-degree night in the tent, verdict: snug as a bug in a rug. Can't beat down for warmth.