I LOVE my serape! It is one of the coolest, most revolutionary pieces of outdoor gear I have ever bought, hands down! I have noticed that it has gotten a little less warm over the years, but I've ridden it hard, and its still pretty dang warm. My wife also has one that has a lot less miles on it. I don't know about zipping a full-size and a medium together, but zipping two full-sized together produces a nice two-person sleeping bag in our case. I keep mine in a Kifaru XL ultralight pull-out. Its roomy enough that I don't feel like its over-compressing it. I pair it with an exped poncho, and a paclight goretex jacket that is a little too big. That way if its really stormin I can put the serape on, and gather it up inside the jacket. Or I can just wear it under the poncho, or whichever combo I need. (I'm anxiously awaiting an HPG Mountain Parka to replace the gore-tex! ;) I carry these three pretty much everytime I hike anywhere in southwest Wyo. I used to think about how uncomfortable an un-planned overnight would be, but now I don't even worry about it. Between the serape, poncho and some well-placed fire reflectors, I think I could ride out a LOT of weather in relative comfort.
I recently bought one of those orange thermal sleeping bag liners from sea-to-summit, I'm going to try that in conjunction with my serape for a summertime sleeping bag system this year. I've spent a lot of nights out in summer with just the serape, most of the time its fine, but sometimes its just a touch chilly, so hopefully this will give me a little more edge.