Recently picked up a pair of nike zoom kaiju's so I thought I'd add my two cents.
Regular shoe size 10.5
Old boots: thirtytwo prospects size 10
Nike Zoom Kaiju: Size 10
Right out of the box, I could tell that the Nike's fit a tiny bit smaller than my old thirty twos. The toe box was very narrow, and the length was slightly shorter. In terms of length, the Nike size 10 was perfect, my toe was pressed right up against the front of the lining so it was tight, albeit slightly uncomfortable, but there was definitely no bending. For size reference (compared to my old thirtytwos) I always thought my thirtytwos packed out a bit too much and were too uncomfortable. I tried on a pair thirtytwos (lashed) size 9.5 but they were way too small and my toes were cramped. The nikes fit like a 9.75, it was the perfect fit for me (in terms of length). I think the biggest thing about the nikes is they have an incredibly narrow toe box. I don't think I have fat feet... although i do have flat feet. When I was wearing them around the house, my pinky toe (and only he pinky toe) would go numb because of how cramped the toe box was.
After seriously riding in them all weekend, it felt like the boot was starting to pack out quite nicely. The sole packs out surprisingly quickly, the toe box is still very cramped but I can ride for 3 or 4 hours without noticing it. Lots of cramp but no more numbness. I expect after a few more solid sessions, they'll feel really nice. (I will update)
Aside from the tight toe box, I'd say these boots are awesome. There were absolutely no pressure points anywhere on the boot. The lacing system is kind of annoying but well worth it for how nicely it keeps your foot in without pressure points. The lacing for the hardboot is nice too, you can lace up the bottom real tight and leave the top nice and loose. The built in J bars are a miracle worker. There was zero heel lift even when I was forcefully and intentionally trying to lift my heel. The boot sucks your foot in.
In terms of flex, definitely a lot stiffer than my thirtytwo prospects although, this really isn't a fair comparison because the zoom kaiju's are a much nicer boot. I should probably be comparing this to a high end thirtytwo boot, unfortunately I don't have that luxury. The boot starts of stiff, but ones you start riding, it starts to give. The forward flex is really soft but the lateral flex is kind of stiff. I suppose this is good cause its forcing me to squat instead of riding lazily all the time.
anyways, this is all I have for now, will update in a few weeks when I get a few more sessions in. See if i can get that toebox to pack out. I definitely don't regret getting a size 10. I know everyone says you should stick to your true size for zoom kaiju's but if i'm paying 300+ for a pair of snowboard boots, I want them to fit after 10+ sessions. I'd say stick with your regular snowboard boot size or half step down from true shoe size and suck it up until it packs out.