I have one.
Great for summer backpacking,excellent quality, nice and compact, light weight, waterproof...
You are welcome to buy mine, very competetive price, only slightly used...
THAT should tell you something
All the above is true, BUT, I use Snugpak Military Sleep range bags, (I am sure they used to be called Code Green), both a No6 Kestrel for the summer and a No10 Harrier for winter. The Harrier won't fit inside the bivi bag without losing some of its loft. If you try to follow the advice to put your kip mat inside as well, you will get a great insight into how a salami feels! I am not a particularly stout person I think it pretty
that Snugpak make a bivi bag which won't take one small/average bloke in combination with their own range of sleeping bags!
Now, they MIGHT have changed dimensions since I bought my bag a few years ago, but you need to check it, probably by trying one out and being ready to send it back if it doesn't fit. The only reason they didn't get mine sent back with a flaming letter was that I didn't get the No10 bag until I had been using the bivi with the No6 for a year or so.