I use 2 layers IX for three seasons, gets me down to about 40 easily as an UQ. I then use a three season Phoenix to get me down to about 10-15 degrees, a mat or some warm window between layers in the hammock for lower than that.
IX is a great supplement, I have had no condensation issues with it as an UQ. I also use 2 layers of IX as a cover or a sock when it is below 0 with it slightly vented, works great. I think IX is not an end all, but used with some common sense and as a supplement it really has a place in my gear.
I have used the above combinations with a down sleeping bag to 14 below. That is my personal record. Twas mighty chilly that night. Heard the trees pop as the wood expanded from freezing.
The World is my country, all mankind are my brethren, and to do good is my religion.