I bought the double hoop Bivi from Terra Nova form their seconds shop. A fantastic buy at a good price. I recently spent a week in it, in mixed weather. I used an ex arm poncho to store my gear under... a brilliant experience.
It is a little cramped inside, but comfirtable to sleep in, if you have to sit out in the rain, thats what the ponch was for...
I did not do much shopping around, I just went for it since it was so much cheeper being a secon (and i still do not know the reason).
One comment i do have is condensation. The hood has a gortex and a mossie net layer. I found that if i zipped the gortex hood up fully, condensation was a bit of an issue, the vent holes just dd not let enough air through... Ideally it needs one at both ends to ensure a throughput of air...
If i left an inch or so of the door open (just the mossie net shut) i got a nice airflow through, and because of the lip over the door, it did not let any water in.
One comment for Terra Nova (if they are listening) would be to put an air vent in the side, probabluy up near the head end, that could be pegged open (like the one at the foot end) with a short line making the whole thing just that much better.
G: