There is definitely some competition: Western Mountaineering for sleeping bags, Nunatak, Jacks are Better, Golite, Enlightened Equiptment, and Javan Dempsey (a forum member who makes custom quilts). I have experience with WM bags, Nunatak and Golite, and so far Katabatic takes the cake. For me, like John V., the contoured cut makes a huge difference. There is a little less girth then some other quilts, but I think its a more efficient use of down (more of it is piled on top of you), and there is less material. If you look at the ratio of down compared to overall weight of the bag, Katabatic stuffs a lot of down into their quilts. My sawatch is only 25.5 oz and has 16 oz down, and with the cord system I can customize how efficient I want to bag. In my experience WM's ratings were accurate, Katabatic's ratings are conservative. Quality and attention to detail better than Nunatak.
1) I think in general these bags are stuffed really well. Western Mountaineering is pretty much the standard for warmth in sleeping bags, and my katabatic is stuffed a lot denser than WM, perhaps stuffed more than necessary, but I'd rather have extra down then extra shell material. I'm looking at getting a chisos, and from what aaron says that bag is not stuffed quite as densely as some of other bags. I'm looking at a chisos with 1oz of overfill, so I can use it for most of my three season fast/light trips.
2) I have a sawatch and if I get the chisos, than I will plan on using the two together for extreme cold. I tried this combination together in the shop, and there was a little bit of down compression in the footbox, but not enough to have a negative effect. If you are going to do a lot of winter trips than you might consider getting a wide version of a warmer quilt like the alsek and then a chisos to layer under that. Some people will also layer a wide sythnetic quilt (like an MLD) over their down quilt or sleeping bag to help keep condensation from getting caught in their down bags. If I did a lot of winter trips, than I'd probably layer my sawatch with a MLD spirit, but since I'll probably get a lot more use out of a UL summer down quilt, I'm looking at a chisos.
3) If you are transitioning from using a sleeping bag, and you shift positions a lot, you might notice some drafts, but these are easy to control. For instance if you like to tuck your knees in completely when lying on your back, than you will probably leave a little opening in the bottom of the quilt for drafts to get in. Solution: don't lie on your back with your knees tucked in. No big loss considering the many benefits of a quilt. When quilts first became popular there was a lot of talk on these forums about learning to use a quilt as part of your backcountry skill set. IMO, you shouldn't need a skill set to go to sleep. With the contoured design and the cord clips, katabatic has pretty successfully dealt with the draft issues. Learning curve N/A.
Hope that helps and sorry for the novel. Considering a sleep system is a big investment, I put lot of research and thought into it. Glad if I can make the decision a little easier for others.