Went to a music festival this past weekend (including Kathy Mattea, Donna the Buffalo, cast of many other talented musicians) with the high temps in the high 90's F and the lows around 70F. Way too hot to use an underquilt at bedtime, but was getting a little cool by early morning with nothing under me.
Before I left, I had installed some of the modifications that JRB sells onto my DriDucks poncho (corner tabs, drawstring), so that I could use it as an underquilt OR a poncho at the festival.
Long story short, it worked like a champ to be a double-duty item. It never did rain, but I did get cool enough to pull the poncho under as an UQ and it was just enough to stay asleep.