I definitely side with Carrow's experience concerning the civilian "Biv Shelters", although my own experience is limited to after-market mil-spec (rather than civilian-focused) gear.
I am quite willing to be the "rebel" in the group and say that the $200 I spent on an UK "Survival Aids" hooped bivvy bag (eg. 1-man tunnel tent) back in 1990 was the best "kit" money that I ever invested. It has given me better "bang for buck" than any of my subsequent investments in a Stealth Suit, CADPAT Recce Smock, etc, etc. That modest investment gave me an OG goretex bivvy bag rougly 10 years before they were issued. Even after the CF bivvy bags were issued, there was no question in my mind that my "vintage" bag was far, far superior to the very basic design of the CF-issued version.
I can't speak to the other "hooped bivvy" models, but I can tell you from 15 years of personal experience that my "Survival Aids" bag is seriously multi-function and totally weather-proof at less than 1/3 of the weight and bulk of the CF-issue goretex "body-bag". Got 5 minutes in a static location? Set it up as a tunnel-tent with the 8 x aluminum pegs and 4 x guy-lines. The bag is suspended off of your body (providing ventilation and lack of condensation), it has a built-in vestibule for your kit, and it features an integral insect net. Don't have 5 minutes for set-up or are on "zero minutes" notice to move? No problem - use it as a standard unsupported bivvy-bag. Regardless, my after-market goretex bivvy is still 1/3 of the weight and bulk of the issued bag, and has the bug-net as an added bonus.
My Survival Aids "hooped bivvy" packs into a self-contained 4" x 8" cylindrical package weighing less than 2 lbs (including the shock-cord aluminum hoops and pegs). It does everything that the issued bag does, and a WHOLE lot more. I have had the issued bag for 6 or 7 years now, and it is still in the original wrapper buried in a barracks box. You couldn't pay me enough to lug that milllstone around when there are far, far superior alternatives for personal shelter.
Just my $.02, as a VERY satisfied goretex "tunnel tent" user dating back 15 years. Yes, the issued bag is adequate. Are there FAR better solutions? You bet there are. And (surprise!) those superior products pre-date the current-issue bag by at least 10 years...... :-\