I shut the heat off a couple nights ago and tried this out. It got to about 58. I stayed nice and warm, actually, I had to take my feet out of the footbox, because they were too hot.
Something I didn't anticipate was how itchy the material is. I need to try washing it a few times. If it doesn't soften up, I might be gifting this one.
I made another out of lightweight fleece I got for $8. It weighs 3oz more, but is way softer. Again, I tried it with the heat off and my feet got too hot (this time it got down to about 53 inside the house). It was way more comfortable, though. I may "vent" the footbox, somehow.
Of the two, the fleece is warmer, more comfortable, and packs much smaller, even if it is a few oz heavier. I think it will be a decent summer solution, until I can afford something very lightweight.