I bought the Overcover when I purchased the rest of the HH supershelter for my Explorer UL this Spring. Since then I've probably carried it 200+ miles in my backpack, but never used it.
Spent 4 days last week up on the Superior Hiking Trail in Northern Minnesota trying to catch the last of the Fall colors. The weatherman told me it was supposed to be warm and sunny...he lied.
The first night on the trail I was setting up in one of the Gooseberry campsites and it looked like it was going to get pretty chilly, so I dug the overcover out of the bottom of my pack and hooked it up.
Setup is pretty trivial if you are accustomed to the supershelter: throw it over the ridgeline, pass the elastics through the end of the undercover and hook to the prussics, then loop the hammock side elastics through the side holes in the overcover and tie them down as normal.
After dinner it got dark pretty early so I crawled into my hammock and it hit me immediately -- it was warm inside!! I was wearing lightweight silk LJ tops and bottoms plus a Powerdry LS shirt, and sleeping in a +30F down bag. After a comfortable hour or so listening to my iPod I dozed off, but woke up at about 11:00PM overheated. Had to take the LS shirt off!
Crawled out next morning and looked at my thermometer: 40F.
The next night I stayed at one of the Split Rock campsites and it got even colder: it was 28F in the morning when I got up. This time I didn't bother with the LS shirt when going to bed, and was cozy warm all night.
By night #3 I was starting to like this overcover thingy. A little warmer that night, about 38F.
After the first night I left the overcover attached to my HH for the rest of the trip. It rolled up no problem in my snakeskins - it was a little more snug than normal, but not an issue to get my hammock, undercover and overcover all in the skins.
My big worry with the overcover was condensation and that turned out to be a non-issue. There was a little dampness on top of my space blanket in the morning, but from what I could tell it was no more than normal. I had no condensation on the interior of the overcover.
The overcover does not seem to seal tightly at the bottom along the edge of the hammock. This is not a Bad Thing, as I kinda liked a little bit of fresh air coming in through the crack at the bottom. The "breathing" hole in the overcover is pretty small, and supposed to act like a bellows when you move at night. This seems to work, but I did appreciate a little extra fresh air at my face level.
Since the trip I used it last night in my backyard and noticed that the bottom openings seem to be caused by insufficient tension on my side elastics. The overcover seems to have some amount of designed vertical tension, and if the side elastics are not pulled fairly tight the overcover will pull them up and create the opening at the bottom.
Anyway, I am a convert. It is pretty remarkable to be able to push my down bag several degrees below its rating with no extra clothes worn. I'd guess that it gave me at least an extra 10 degrees of warmth. I'll have to play a little bit with bringing a thermometer into the hammock and check the interior temps. If you consider that it weighs next to nothing, and takes up almost no space that is a huge win.
Caveat: of course it does nothing for bottom temperatures. If you have insufficient insulation beneath you, you'll still freeze your butt off.
Anyway, that's my report. I'd appreciate hearing from others who have tried or are still using their overcover to get any additional usage info.