When you want warmth without weight, you want down. When you want compressibility, you want down. But, down isn't typically very cheap. at all. So when I got a chance to get a close-out price break on a 2014 Kelty Cosmic Down sleeping bag, I seized my chance. I already have a good rectangular synthetic bag, a LL Bean bag I got from my grandfather. But that one's not very compressible. Actually it takes up a heck of a lot of space in my car camping kit. So, I started out using a Polar Fleece sleeping bag that was probably only good down to about 50F or so. It actually did okay by me, when I traveled west last summer, it served me just fine with my hammock/tarp/foam pad combination, but we were on the northern coast of california, and it stayed within the 40-60 degree range the whole trip. I wanted something light, something compressible and something affordable.
The Kelty Cosmic Down bags are considered to be entry level down bags, and that's right where I'm at. Their 40 degree rated bags are even cheaper, but after spending a night in the Wisconsin spring in mine, I'm glad I went with the 20. It's also a mummy shaped bag, and you can near completely ensconce yourself in it. It's a little unusual for me, I'm used to a larger bag, and though I'm not stuffed so tightly in the bag that I defeat the insulating properties, it does restrict my movements and positioning a bit more than I'm accustomed to. The other thing about down is that when you compress it, you lose a lot of the insulating power, so you'll really want to have a foam pad or inflatable mat to sleep on.
The good? This baby is warm. When I first went to bed, full of heat from digesting my sizeable dinner and soaking up campfire heat I was actually on the verge of being uncomfortable I was so warm. But as the night wore on, and temperatures dropped, I was very glad for the bag. I'm a cold sleeper, in that I do not generate much bodyheat while sleeping at all, and the Kelty Cosmic Down did a great job keeping in what I did make. It also compresses very well, fits right in the bottom compartment of my pack. Weighs very little, might not fall into the ultralight range, but it's light enough for me.
You'll also want to make sure you don't store a bag like this compressed. If you store it all squished flat, it won't re-loft very well, and it'll do a poor job insulating. It's also good to make sure you set it up long enough before bedtime that it can breathe a bit and loft up. I've got mine hanging over a door, some dangle them from the rafters of their attic or similar. You can also get various storage bags that don't compress the sleeping bag, they just protect them in storage.
Would I recommend it? Yeah, it's good if you need the lightness and you're sure you can keep it dry. Synthetic solutions are better in that they can stay warm even if wet. But for weight, Down is great. You can get them on Amazon here. REI used to have them, but doesn't seem to carry them at the moment.