My thoughts - just received mine yesterday in Canada

Somewhat unorganized thoughts:

• The box was extremely heavy, I used a trolley to bring it up to my apartment, then I couldn't lift it alone so I had to tumble/roll it over on its side to get it through the door to my apartment then unpack it there instead of in my office area.
• Assembly was pretty easy, all the packaging was recyclable (lots of origami'd cardboard pieces), but it took me so long to break down all the cardboard and the box to toss it, which I had to do right away cause I needed to reclaim the space in my tiny apartment.
• I was confused by the front screw connecting the arms to the recline, it was really awkward to align the little flappy thingy to the screw hole because the arms kept bouncing back away from the fully pulled forward position. They could certainly improve that part of assembly by locking the arms in the forward position with some zip ties or something to make it easier to screw in alone.
• Context: I'm 6'0 ~210lbs. I'm coming from a ~12 year old Aeron that I got ~6 years ago second-hand (from a guy with a storage unit filled with office chairs he would collect from office closures/clearance)
• The seat is still way too short (48" version), I have like 8 fingers of room behind my knee to the front of the padding. Unfortunate that didn't lengthen the seat pan and only extended the foam pad, means tucking my foot under doesn't feel that good, one thing I was hoping I would be able to do again (haven't been able to since I've had an Aeron whose scooped hard plastic sides completely prevents - I used to tuck my leg on my old DXRacer gaming chair that's now long-gone)
• The casters are excellent, very smooth on my hard laminate floor
• The stem/canister sits further back in the chair than I'm used to (not directly under the center of the seat, but right near the back of the seat pan) because the back of the chair is heavy with the whole lumbar mechanism, this means the legs are well "behind" my feet so putting my heels on the legs doesn't feel like a real option. Not really a negative, but just something I noticed quickly.
• I have a relatively curved spine/neck (I've always had poor posture), the default straight-back position is not comfortable for me and I would want it slightly reclined by default, but it takes a bit of force to push the back to initially recline, and there's no say to lock it to that position, so I can't just sit down and feel good, I need to sit then push back and I always feel in tension that way.
• The distance between the 1 and 2 positions of the recline limiter (1 being no recline, 2 being some recline) is way too much, I would want a 1.5 position which would be the sweet spot for me (if only it was also a lock instead of limiter). This means to be comfortable I need to overly tighten the tension knob so I don't just fall back to the 2 position, and my back has to be in tension to keep it in that position.
• The seat pan does not tilt along with the rest of the chair, something I've grown used to with my Aeron, leaning back would kinda keep me in a scooped position which I found quite comfy. The Omni doesn't do that, but the seat being fabric means I don't slide off like I might with the Aeron if it didn't also tilt since the mesh is slick.
• The back being fabric plus the fact I need to recline a bit every time, means that the seat has high friction with my shirt, so my shirt always rides up my back and that feels really uncomfortable. If the seat back was more slick then my shirt wouldn't ride up since it would have more friction with my skin instead, which is how it is with my Aeron.
• I can't really "fix" my shirt after reclining either cause if I try to lift my back off the chair, it also immediately un-reclines, there's no way to lock the reclined position, it's only a limiter and no lock. This sucks for me. So now I have to grab the back of my shirt with both hands before resting my back on the chair to not be bothered by it.
• The arm rests are way too firm for me, the Aeron arm rests are so plump in comparison, even being 12 years old and worn down.
• Not having a way to lock the arms in a certain position, especially for the forward/back slide or the in/out tilt is a huge problem, if I tilt in the arms and then lean over on one arm to adjust my butt or just to lift/push myself out of the chair by my elbows or my hands, the arms reset to the out-tilt position and I kinda "fall" and it's jarring. Makes the tilt basically useless. Having to reset it every time I sit down is really stupid.
• As others have said, the lumbar arm pushing up means the top of the chair lifts away from the upper back, so you're just trading lumbar support for upper back support, that's really unfortunate.
• The massage thing is obviously a gimmick but it's kinda fun, but not a killer feature.
• My butt is already sore after ~2 full days of use, certainly less comfy than my Aeron in that respect. Disappointing. Largely cause not enough length in the seat pan I assume, so weight distribution along my legs is focused more on my butt. Similarly to the back, fabric seat means high friction with my pants.
• The headrest doesn't go forwards far enough for me, I wish it would have like an inch more to meet my head where it is with my curved spine.
• I use an aftermarket headrest for my Aeron https://www.amazon.ca/dp/B00L2IJNMK (though mine isn't like the one pictured at the top of that link, it's the "Graphite Fabric Cushion" one pictured further down the page), I really like that it curves around the back of my head so I have some support if I'm fully reclined from my head falling over to the side, I could use it to take a quick nap with my feet up and know my neck wouldn't hurt when I wake up. With the Omni headrest, it's pretty flat on the back of my head, so nothing to stop my head from rolling over to one side. Also it flops over side to side a bit cause the tolerances on the sliding parts aren't super tight, feels too loose.
• The footrest is really unimpressive, the hard plastic shelf is no better than the floor. I use a semi-circle shaped foam/velour footrest (keeps my feet warm, especially when my floors are cold in the winter) plus a 1ft x 1ft box with a small pillow on it when I want to kick up my legs, and that still feels like a better setup.
• Minor thing but it's too easy to randomly touch somewhere under the chair or the armrests and end up touching a greased up sliding part leaving my fingers icky, those parts should be a bit more closed off from touching (smaller slits instead of being finger-wide gaps).
• I've already accidentally hit the massage mode button more than once, like if I'm trying to get at my left side pocket or scratching my hip or whatever, and since it's a toggle it immediately throws the lumbar support off from the sweet spot I had it at. As a workaround, I could just unplug the battery I guess if I don't plan on moving it, but I feel like the massage mode should probably be a "hold 2s to activate" kind of thing, or at least be a stiffer button to avoid being accidentally bumped, or just be positioned somewhere else entirely (left side under the seat, above the battery slot?) That could also simplify the arm rests (they have a cable going through the arm stem to connect the buttons to the electronics in the seat pan).

So uh... all in all I might need to go back to my aging Aeron, possibly buy a brand new Aeron soon, and sell the Omni or pass it off to a family member or friend. TBD. Too many problems for me for it to be ideal for me.