Well I recently returned from my Yosemite high country trip. I knew going into this that it was going to be cold at night. It usually is near 10K feet. Proper insulation in the HH was a major concern especially since I didn't want to spend the cash for a quilt. Here is what I had.
Explorer Ultra Light
Walmart Blue Pad
Moonstone 20F bag
LL Bean Down packable jacket
First off as you may know the Sierra has some BIG trees. Really big. In fact there aren't many small trees and one of the biggest challenges was finding trees small enough to set up the HH's. I had a set of 42' Tree huggers but in most cases they were useless. We managed to find some smaller trees at every stop but sometimes it wasn't easy.
First night it was warm in the low 40's. Never even put on a hat. Besides the bears sniffing around all night I was very comfy.
Second night the cold air moved in. Got into the low 30's. Put that hat one at some point in the night and my feet got a little chilly. I was using the fleece blanket as a pillow and had the down coat in the HH but it wasn't being used for anything. I was inside the moonstone mummy bag. I was wearing fleece pants and a fleece pullover. My feet were extended a foot or so over the end of the Walmart Blue. No insulation under them and I felt it.
Third night (9200 feet) was cold. 20F easy. My feet got really cold so in the middle of the night I wrapped the down coat around my feet. My back was fine. There was no wind and I may have been a problem if there was. My feet never really warmed up. I should have started the night with the down under my feet.
The next morning I went to take a drink out of the Nalgene and as soon as I opened it it started to freeze. It basically turned from a liquid state to solid in 5 or 6 seconds before my very eyes. Crazy.
4th and 5th night was in the Valley at around 4000 feet. It was in the high 50's at night. Slept with the rainfly off and gazed at the stars while listening to Dark Side of the Moon in the Ipod. The backpackers campground has about 25 sites and only 2 can hold a hammock in the trees. Big trees everywhere.
6th night slept in a hotel near the ocean and it was the first night on the trip I woke with a sore back.
Bottom line: The HH is not any worse than a tent in 20F weather. The Blue pad worked fine. Minimal condensation issues. Easy set up most of the time and was a ultra comfy way to backpack. I'd be concerned it there was a wind storm. The rainfly didn't cover that great. Works ok in the SN where they basically get no rain June-Sept. Money well spent. Now I need to figure how to set the HH up in my room! No more tents for me.