Ok, I've only used the bag for two nights. I used…
Fill: 750 down
Temperature Rating: 20 F
Weight: 2 lbs
Price Paid: $165
Ok, I've only used the bag for two nights. I used the bag in conjunction with the Big Agnes Wedgie bag expander. The Zirkel has a little more room than other mummy bags, but because the pad is attached, it limits movement a bit (e.g. the bag won't move with you if you try to bring your legs up, it's stuck to the pad beneath). That's why I added the Wedgie.
The Wedige did what it was supposed to do--I didn't feel constricted and I could move around comfortably in the bag. But it may have let some cold air in at the top- and I don't know what the temp rating for the expander itself is. Temps dropped to around 25 F and I was a little chilly, despite the bag's 20 degree rating-and I'm not exactly a cold sleeper.
Without the Wedgie, I would have felt constricted (I move around a LOT while sleeping), but I may not have been cold. Hard to tell. Hence, the 3.5 rating. The bag itself appears well constructed, it's relatively lightweight, and the pad, while heavier than the Therm-a-rest I had been using, is much more comfortable.