I don't have the ID eVENT Crysallis or any previous iteration of the Crysallis. However, i have some bivy sacks that are head opening like the Crysallis.
There are to be sure advantages to the Crysallis as you stated. However, i really don't like this type of b-sak as much as a Bugaboo/South Col (or similar) footend FACING opening bivy (bivy opens at headend, but the opening FACES the foot end, e.g. the Uni or Lightsabre, etc). Maybe it's just that i don't have any headend opener with a stiffened wire dome?
However, I find that when it does rain that i really have to close it up more than an opening facing the foot end. Sure, since it's not staked down, typically (though i may put one in the foot end to keep it from blowing away when i'm not in it when there's heavy winds and my UL gear inside can't weight it down enough to keep it from blowing away; nature call is one thing, running through nature at night chasing my bivy is another thing, however) i could sit up, get my arms outside and shift orientation, but...
Sometimes when it rains when using a footend FACING opeining b-sak, due to the overhang, i don't need to make any adjustments (unless it's unzipped down to my waist). Just continue to sleep away. In my case, counting on waking up due to some rain, even heavy, is 'iffy', at best. I sleep like a 'dead man'. In bear country (which i don't believe i ever really am in - even though 92 black bear sightings in the tiny State of CT in the Spring of '06), i'd probably wake up the next morning in the stomach of the bear!!! i sleep very soundly. I'd be soaked b/f i wake up. I've actually dreamed that i was taking a shower at home when i've been rained on in the past!!! Then it gets cold as i get wetter and waking from my dream, real life/reality becomes a nightmare!
I do have some OR clamshell/crocodile head end opening bivies. These work better, IMHO, than the one b-sak i have that, minus the wire stiffener, is cfg'd like the Crysallis. However, even these i don't like as much as a footend FACING opening b-sak/b-shelter. One advantage is that those whose opening face the footend often have an overhang so that i can still place the opening close to my mouth and exhale out of it even in the rain - this is when it is even more impt NOT to accumulate exhaled water vapor in the b-sak since given the outside RH being so high, it won't pass through the WPB shell as readily. The headend openers i have, when it's raining, require me to move closer to either side and turn my head to the side in order to attempt to exhale out of the b-sak as much as possible, which, in my case, OthersMMV, is NOT how i sleep (i sleep positioned like a dead person in a casket/coffin).
Lastly, ingress/egress in rain allows water entry. At least the Uni keeps most/all of the water out of the headend.
Some feel that a b-sak cfg like the Uni is the easiest to get in and out of (though shorter ones like the Lightsabre are supposedly more difficult for taller individuals; personally, i have no problem since i'm so short). Others like open a head end facing opener nearly all the way, sit down, pull their legs in, and lay back, while pulling the shell and/or netting over them as they do so. To each their own. I'm not sure that there is a best here.
I definitely prefer side zippered openers to mountaineering cross chest openers. All i need to do is throw my back out wriggling into one of those and i'll never get out in the morning - it would become my own personal little 'time capsule' of sorts.
I think that i may give the ID eVENT Crysallis a go and have already, some days ago when i read about it, 'Bookmarked' it for possible future purchase.
Considerations vs. the ID eVENT Uni:
Definite Pluses: no staking, so therefore simpler; way more bug netting - Hoo-yah!!
(tentative)Minuses: head end opening, less head room for reading, etc.
I'd really like to find someone to shorten and re-tape my ID eVENT Uni (or the ID eVENT Crysallis - IF i purchase one) since they are both EXP lengths, i could save some weight since i don't even need 5' 7" of 'fit', much less 6' 7". At 5' 7" to 5' 10" overall 'fit' length, i still have plenty of space at the ends for gear storage.
Don't know why ID doesn't make a shorter length. I've called twice over the last 2.5yrs or so, and asked them. Ans: No. They 'say', so to speak, on one of their webpages that they do custom orders, but they explained to me that is for Police/Military/SAR customers ordering in Qty, not single items.
Sorry, more than you bargained for, no doubt. Take what you need; leave the rest for eventual decomposition.