My Jerven bag lives in my daysack and is used when I stop for lunch etc. on day hikes, and it's great for that. Also use it as a cape when the weather is showery; it covers my pack as well. (For heavy rain I take my Rohan cag.) Not overnighted in it, although did take it plus the lightweight liner with me when day hiking last winter in the snow in the Black Mountains, in the Brecon Beacons NP, as a precautionary measure. (I'm generally a solo hiker).
Also used it as a "bathroom" when having a full body wash in order to avoid scaring the other campers on assorted campgrounds.
Tried it inside out in southern California in serious heat last year to see if the reflective surface would keep the sun off. No. Needs to have a decent airgap and airflow between me and it for that, so would need to rig it as tarp rather than bag.
Looks like I need to get me square tarp. Have numerous rectangular ones.