As a guy who has always loved the D-series, this has been an interesting journey. Now that I've dived the D420 hard for a week, and have gone through some of the trials and tribulations of what seemed to be a balky regulator, I've come to some conclusions.
Is it better than the old D series? Definitely yes. But the answer is complex.
Service: Now that I've figured out how to service the toy properly, I'm feeling a lot better about it. Certainly, it's a piece of cake to physically disassemble and reassemble. That's something that the original D-series could never claim, with that rubber cover! And despite my real misgivings about finding a polymer orifice inside when I first opened it up, it's turning out to be very sturdy. In fact, after trying to "improve" the knife edge on one reg with a little handwork, I think the polymer orifice is actually sturdier than the old metal ones! The metal knife edge was so delicate, and the seat material back then was stiffer, it didn't take much to turn a D-350 into a pig, because of the spring pressure and high cracking effort required to overcome a nicked orifice. That's why it was so much fun to learn how to hand restore the metal knife edges. But that will be an unneeded skill with these new polymer orifices.
I'll be interested in attending my next Service Seminar with Scubapro, to find out if they are teaching the "tricks" I've had to discover by myself, since the service manual is rather disappointing. That's because tuning is not hard, but it's just not as intuitive as with the old barrel/S-wing poppet design. And despite the similarity to the center-balanced poppet of the D-400, it tunes a little differently.
Performance: Just like the original D-series, the short flow path from poppet to mouthpiece makes for fantastic air delivery! My dynamic testing shows great stability at high flow with no crossover.
But what about the loss of the fantastic coaxial diaphragm/exhaust valve? Well, that's where there will be lovers and haters for this reg.
There is no doubt that the original D350 would perform the same in virtually any diving position. Looking up, upside down or flat on your back didn't make any difference, because the case geometry fault was 0.6". Having a reg that you could tune hot with no penalty was pretty cool!
But the original D's weren't perfect. With a relatively small exhaust valve, total work of breathing was not fantastic when you included expiratory effort, especially at depth. As reg design has gotten more sophisticated, things like the S620Ti and the C-series made a point of lowering expiratory effort, even if you couldn't easily tell outside of an ANSTI machine. But nobody could beat a hot-tuned D for inspiratory effort.
And that's where the D420 is a departure from the original D-series. Getting rid of the coaxial exhaust valve will create haters. No doubt about that. But what the change has done is really teach me what case geometry "fault" is all about. What Scubapro's engineers did was to take advantage of CGF, and use it to improve breathing effort in the standard diving position. This reg has a diaphragm that is lowest in the water in the standard diving position, compared with the exhaust valve. That means that the reg's interior pressure is "preloaded" by the extra inch of depth compared with the exhaust valve. What I believe is happening is that that preload gets the valve one inch closer to opening, so that your actual breathing effort in the standard position is cracking effort minus 1". So if you're at the bottom of spec range at 1.2", your required effort to open the valve while diving is only 0.2"! But that also means you can't tune this reg hot. I tried that in Key Largo, and the ease with which it would freeflow during S-drills lost me a fair bit of air and dive time until I retuned it. Tuning hot beyond 0.1-0.2" less than spec gets you nothing but a very twitchy reg.
Of course, if you roll onto your back, you lose the advantage of CGF, and it takes full cracking effort to open the valve. But I'm guessing that the SP engineers felt that that was a reasonable tradeoff to be able to put a bigger exhaust valve in, and in a better position. Because I believe I really CAN feel easier exhalation compared with the D-400.
All told, I'm loving this new reg. It feels sturdy, and it's a breeze to take apart. It delivers huge flow, and was effortless at 120'. Time will tell, but I'm a fan for now. Can't wait for the pair of Mk19/D420's I've ordered for sidemount. Hope the 'Rona doesn't delay delivery from Italy.
I see no issues with environmentally sealed...and would prefer my 1st stages had that feature....but...I dive pretty conservatively and my maintenance is annual so not a giant concern....just love having the extra protection 
