Hi,
I've got one of these and I really like it as an overbag.
I combine the bivy with a Rab Top Bag and a Thermarest Womens Prolite 3, all inside (with me of course). There's plenty of room to move, and the bathtub comes up nice and high. I don't have the bug net. I usually sleep on my poncho as a groundsheet.
I slept in it recently through an amazing thunderstorm in the Pyrenees, and still felt cold underneath. I ended up putting my sitmat (a cut down Ridge Rest) under my hips, and my pack under my legs, and whatever else handy under my torso, so I'm not so sure about the silver insulating properties- it might have been the weather… or the thermarest.
I did have a tiny bit of condensation at the foot end, but nothing that affected the performance of the bag. I think condensation has more to do with what you wear and how you sleep, but the bivy was very breathable, moreso than the goretex bivy it replaced (800g vs 160g).
I was in my Shangri-la 3, and had lots of misting through the night due to the exteem weather, which the bivy handled really well. In fact it was dry the whole time. Don't undersatnd the science here, but it works.
In summary, I really like the Titanium Goat Bivy- it works perfectly as an overbag, and keeps off the spindrift and misting from the sleeping bag very well.
I hope this helps