I am the ultimate cold sleeper. I need the ultimate sleeping bag when temps drop below freezing. I recently bought the REI Habanera Women's Sleeping bag to see if its up for the challenge. The sleeping bag I've been using for the last year or so is the REI Sub Kilo +15 . It has 750 fill down and is super light and awesome but just doesn't cut it when temperatures drop below ~35 degrees (for me anyway).
I chose the Habanera bag after tons and tons of research and comparison. A couple big things stood out for me: the price (I paid $209 on sale) is great for its rating and it is EN 13537 tested. It's really difficult to compare sleeping bags or trust the rating without the EN test offering some sort of metric.
A lot of companies over estimate the rating of their bags (North Face seems to be one of the worst). For example, this North Face 0 Degree Snow Leopard bag claims in the title to be zero degrees, but the product listing shows its EN Rating at : Comfort: 25° F / Limit: 12° F / Extreme: -22° F . Extreme basically means you won't die if you stay in it for 6 hours at those temps. So it's pretty crazy they advertise it as a zero degree bag.
Check out the EN Rating tag at the end of this post. I still consider it a fuzzy guideline for comfort level (its too subjective), but compared to other bags I've looked at this is the best bang for your buck.
More to come when I actually get to use it.