I have a 1960-61 US Divers DA Aquamaster and a 1960 vintage US Divers Mistral. The Mistral actually breathes very well and I use it pretty heavily for fun dives around the boat this time of year when the water is warm (74 degree, at the surface down to about 35 ft.)
It is a nice change to zip around in a T-shirt, a single tank, backpack and horse collar BC. Once you do that and see the speed potential, you find yoursel getting very serious about streamlining your "modern" scuba gear.
I accessorize with a Scubapro vintage depth gauge (with markings to 500') that I bought new old stock this year from a dive shop going out of business.
Clearing the hose is a little different as even with a newer style mouthpiece with wagonwheels and mushroom style check valves. The effort required to clear is substantially higher than with a single hose reg and if short on air, you end up wondering if you are going to have enough air to do it. You don't get the same gravity assist you do with a single hose second stage.
They are also a bit noisier in general with the air from the tank going through the hose on the right and the exhaled going through the hose and butterfly style exhouat valve on tyhe other. And all that air tends to float you a bit more head up in trim if you stop moving. But that's ok as they breath better in that position.
They breathe a little harder than new regs and breathing resistance varies with body position. As soon as you use one, you understand why Mike Nelson kneeled on the bottom or was upright in the water so often when doing stuff (and waiting for someone to swim up and cut his air hose) as a double hose reg is easiest to breath when in that position. If you are looking up or are upside down they will force feed you air, while in a normal swimming position they are a little harder to breathe. Head down, they can be down right hard to breathe from.
Leaning back is not an option on the surface (the position where a horse collar BC wants to float you) as it will cause the reg to freeflow, so you end up huigging the horse collar and leaning forward on top of it.
There are reproduction oval masks available along with new hoses, mouthpieces and butterfly valves for US Divers regs and the various copies.
Lot of fun though and everyone should have one! Fish will swim right up to your mask as they peer around from behind the cover and concealment of your head to watch the bubbles