I don't have one but have, inspected one for myself.
the system seems pretty sound,
zips and fasteners are good quality, the sleeping bag poppers into the bivy bag which means it's easier to use, ie, the bags don't twist at night, an extra liner will popper in as well, i believe you have a choice of popper or zip or both to close the whole system, which would make it very weatherproof and ideal if one of the closures fails.
if you zip up completely you will get condensation problems, the system dosen't seem to have any storage space for a bag so you would have to make provision for proofing your bag!
all in all it seems pretty good, 10 out of 10 for design but as mentioned, size and weight could be an issue!
how you would cope with changing clothes, or making breakfast from your bivi bag (whilst still inside) could be tricky as it seems a tight fit(sleeping bag sized!
goretex as a fabric will always keep you dry as long as the fabric is looked after and it gets re-proofed regulary!
my best suggestion is play with one!
get it out in the shop, get into it, see how small it goes, try all the zips and poppers, (you'd do this with a jacket!), remember you'll wish you had if there's something wrong!
:roll: