I've had the Dark Star for a couple years now. It is an absolutely massive and heavy sleeping bag. I asked my old man for a good winter -20 or -30 bag for the birthday and what showed up was --The Dark Star -- LOL. My personal experience is that it is awesome. It is big and heavy but very warm and comfortable. If it is too hot you learn to manage the heat by simply not zipping it all the way up, which I do quite often. If you are fully zipped up it is capable of the rating that it claims. I use it for a huge assortment of camping, basically any time it will be particularly cold and my small 20 bag won't cut it, but even in fall and spring and in southern Utah in winter, times where it rarely gets below 10 F. It works very well for me, I am 6'5, fit in it perfectly. Again, I manage any excess heat by venting the bag. It would be a great base camp bag though particularly bulky. If size is not a big deal then use it. If you are a winter expedition on foot or limited space there are better bags to get. As with anything though, you always get what you pay for and it is a decent deal as well.

Used the sleeping bag in -15F camping on Lake Superior North shore. Its rating is for -40, and that would be a much more appropriate temp for this bag. It is incredibly warm... way to warm for -15F.
Also, I felt the bag was a little short for me. I am 6'3" and have the long version, but my toes were still right up to the edge of the bag.

I used it a 1 week "expedition" on Monte Bianco with a "base camp about 4400m (we were testing other equipments for a Pamir expedition) with a CAMP XP2 tent. The temperaturs were constantly about -30 - -35C (in winter). It was incredibly warm, but it's difficult to pack it. It fill up to 1/2 of my backpack (80 lt.!)even using its compression sack, and that's a problem. So in my opinion it is a sleeping bag only for serious expeditions (artic or high altitude) or for summit pushes if you want an insurance in case of a bivy on the way down without tent shelter possibility. Absolutley NOT TO USE if you have to pack-unpack every day (need about 15min. to pack it - if you are used to do it...)