I started with a British Army DPM Goretex bag, it's lovely, breathes really well, very roomy, nice camo, but weighs a ton. These days I use the Alpkit Hunka XL, which seems to breathe pretty well, but is about half the weight. IMHO, by the time you're looking at hooped bivvi bags you may as well just get a tent, chances are it'll be more comfortable, and possibly lighter with modern tech.
Its a while since I've used a bivibag.
My first was a Wild Country, Terra Nova Hooped. (Great for writing orders from).
Regularly used the issue non hooped which is big and roomy.
The negative to the two above is that they are heavy; bomb proof but heavy.
All bivibags require a large poncho to do admin under. They also have a limit if its rains day in day out as, even with the best camp admin discipline, eventually too much ends up wet. 3 days about the limit with no break in bad weather. Summer, good weather then they are excellent. Midge nets can be important at some times in the year, and here bivi's aren't great. Synthetic bags compliment bivi use well as they shrug off moisture, internal as well as external, better than down; but add to the bulk of any system (don't get a winter bag for summer use, but do go one season higher than expected).
I fancy a Rab Ascent bivi in eVent fabric as eVent is more breathable and just as waterproof as Goretex. Might save a bit of weight too. I actually want a central zip eVent bivi, more as a down sleeping bag cover as a bivi.
Much depends on your budget. I prefer my Terra Nova Laser II tent now. Vastly more room, better protection, for a similar weight. Just not as bomb proof and high ticket price.