I have been a regulator service tech. since mid 80's and a "factory" trained technician on the Poseidon regulators since late 80's and have seen the change in the Poseidon distributorship in the US during this time. I have sold and service Poseidon regulators through my dive center in NY during the busiest times for Poseidon in the 90's and early 2000's when Poseidon was the advanced/technical diving standard. I have also owned several Poseidon regulators since the mid 80's. My service department was one of the busiest service departments in the area where we serviced most brands. Poseidon regulators are finicky and difficult to get a service technician who is capable in their service in most locales in the US and the world. They are like the fancy Italian fast car, they work great when they work but they need a technician living in the backseat.
I am surprised you said that.
I have been using Poseidon Cyclons since the early 90's [1]. Despite some awful abuse, they have been excellent. I had one season where if they weren't in the water diving, they where in the back of the car, making it very ripe. I don't think anything was washed or rinsed until the October, other than the odd freshwater dive. (Not to be recommended).
The only issues I have ever really had is one particular first stage that has had issues with the diaphragm after servicing. The two shops that have serviced it acknowledge that it is assembly issue. Once assembled, and tested, it operates flawlessly for years.
The only other issue is if you fail to rinse the second stages, they can get a little temperamental. These days I find it quicker just to strip them down every 12months, clean them and reassemble them, they are so simple that it's a 5 minute job.
I put all 6 of my cylons in for service in January for the first time in 5 years. The main reason, I only generally use two for bailout cylinders for the CCR, and one of those is seldom used. The rest had been sat in the garage un used for a few years since the last time I did any twinset diving. My express instruction was, if any are damaged, or not in a good state, write them off and strip usable spares out of them for the rest. All 6 came back, with a glowing report. The downside, I had to pay for six services and six service packs, when I only expected a bill for 4.
I have to say, I also have some Apex reg's, one the single cylinder rig for teaching, the other three where my late partners, two for the twinset, one for the single cylinder. These also have been trouble free, but not subject to the same level of abuse as the cyclons.
I've never liked the Jetstream, to savage a delivery of gas for my liking. I have also seen a lot of users struggle when clearing them, I think the internal volume is a problem with the savage gas delivery. It appears to force water into the back of the throat during purging.
The newer Extreme I have never used, so can't really judge.
To be honest. its a shame no one else does an ambidextrous regulator these days. Oceanic used to do a nice one, but its long since been discontinued.
Gareth
[1] Note I dive CCR almost exclusively these days, so in recent years they have only ever been used as bailout regulators.