I've got an o2ptima and love it. But like everyone here who loves their rebreather, there are pros and cons.
Pros
OTS lung MAVs are just drysuit inflator valves, easy to replace if necessary.
Same head can be used with OTS lungs, BMCL, or with cho2ptima.
Kydex cover reduces damage to the stuff on the back, and looks a lot better than the older fabric one.
ADV breathes great, and is easy to adjust to how you want it to fire. Also easy to move to exhale lung if desired. On off is easy to find so it's easy to shut it off when you want it off.
To some the cartridge is a big plus. For me, the ability to use sorb is a big plus.
Cons
Electronics pods. They flop around when storage. Stock, the HUD is on the right, but the pod is also on the right. I flipped HUD to the left for viewfinder use and cross the cables over the back, much better routing.
The Dive Can plug connections are great, and it's not an o2ptima specific cable, but I do wish that the screw on things to keep them connected, came with one extra male side to protect the pins better in storage.
Bottle connections. The cam bands are high quality, easy to use, hold stuff securely, but I always have this nagging feeling that something like the kent tooling quick disconnects would be easier. I don't see how these could be added easily to the existing setup but if someone out there has done it, I would love to hear about it.
O2 tube from the head is annoying. You could remove it for storage, but that's just inviting you to lose it. I have trimmed mine down so it enters the tube but doesn't run the full length.
I wish someone sold a cover that would help me keep dust out of the head for storage, and another one that would let me pressurize the head for linearity checks at home. Both would be easy to mill out, but probably not cheap.
The design mostly precludes this, but I do wish you could increase the sorb capacity. I know people who push the sorb out and are doing long dives on the o2ptima, considering that the last hours are deco with low exertion, but sorb is cheap and my life is not. The only way to do this would be to make the system wider--which it could go wider but at the expense of more drag, unlike vertical canister designs. Or, convert your head to mCCR with a needle valve and trim the head depth, to allow a longer (wider) canister without additional width. But at that point, you might as well just buy a rebreather with a vertical can.
I don't hate the o2 connection, some use a regulator quick disconnect, but a locking one would be way safer.
The carry handle is black fabric, and now I have the electronics crossing right by it. I wish it was a red fabric so it was easier to see.
Overall I'm very happy. I've never had any issues with water in the head, the counter lugs and water trap do a great job. Modifying for offboard dil was easy. Changing the stand height to use different size tanks was easy. The stand does a good job of holding the unit. It's a very easy size, with great work of breathing.
