I have the hunka, and the AMK SOL Escape Bivvi which Frank suggests.
I took the alpkit when I did Ruska last year. I took the SOL on the TCR.
IME the breathability of the SOL is poor compared to the hunka. The reflective material isn't as warm as I'd like. I slept in a grave yard in Germany, with a half bag and a down sleeping bag, on a think CCF sleep matt, and I was cold after about an hour or so. I don't understand these people who say they can use a bag like this on it's own, or with just a liner. I must sleep really cold compared to everyone else.
If it's not for a lot of use, as a Just In Case™, then the SOL is OK. If you're expecting to make a lot of use of it. Then the hunka is the one to use.
Oh, and to throw it in the mix, if weight is no object, then the British Army Goretex bag remains the best performing bivvi bag I've ever used. But it's waaaay to heavy. So it just lives in the kit box now.
J