There are a few reasons why Poseidons are no longer considered to be good regs for Tech diving by some tech divers, not least of which is that the Odin is an upstream reg -- a bad thing in a low-oxygen environment.
Add to that high intermediate pressures up to 180 PSI and you have a reg that will dump the entire contents of a cylinder very quickly. There just is no reason for these high intermediate pressures. A properly tuned, quality reg will breathe very well at depth during demand.
There are other reasons I've heard, "wet" breathing when not level, which when passed off which could be a serious problem in an emergency. Not to mention very expensive maintenance and hoses and parts being proprietary. I paid $100 for a long hose some years ago! Some don't like them because you can't pop the cover on the 2nd and clean out debris if needed.
A few years back they were the reg to have for tekkies, they breathed well at depth and were generally considered to be reliable, but times change.
There are so many better, safer and overall, often less expensive regs that have become the standard for safer tech diving. I love my Beuchat VX-10 and the Apeks are said to be very good. There is a new generation of Atomic Tech regs coming out that have been redesigned. The old Atomics holes in the front of the 2nd stage were too large and contributed to free-flowing.
I've been using my Poseidons for stages, but I'll be selling my Odin and Cyklon 5000, I'll give ya'll a good price! <g>
JoeL
. Besides, why buy used when you can buy new so cheap? I bought that older Zeagle for $300 before I knew better. I do have an older Sherwood Magnum with octo, that I'm considering selling 
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