I got to try my wifes new Delta 4 with FDX 10 first stage tonight at the pool. She switched the mouthpiece to a Sherwood wisdom mouthpiece after trying out the standard Oceanic mouthpiece which she didn't like.
Pros
Ultra smooth delivery
Very dry in all positions
Hose configuration is similar to the ATX 200 and is very comfortable.
The standard swivel hose is much, much, much, better than the Atomic as it is not nearly as stiff as the Atomic swivel when pressurized.
Very light weight second stage.
Ease of breathing at depth, I don't know yet as the pool we were in is about 17 ft deep.
It is about $100 bucks cheaper than a S600 and about $200 cheaper than a B2.
Comes with blue, yellow, and black second stage color themes. (This was my wifes favorite option!)
Cons
Original mouthpiece not to comfy and seemed to cause a little seepage in awkward positions.
Slight squeek on inhale from the 1st stage. I believe this is the d.r.t. resonating as air passes by. (Not very noticeable) Probably something a little christolube would cure.
Bubble interference not up to par with an Atomic M1 or a Legend.
The reg she replaced was an Atomic B2 which she liked but other than bubble interference and possibly a better breathe at depth this seems like a more likeable reg.
Gillty