I just purchased and flew the aircraft. I have to say, I’m more impressed than I expected to be. There are obvious drawbacks. Though Carenado price, you’re certainly not getting Carenado visuals here. But as every virtual pilot knows, at the end of the day the visuals (though important) do not matter as much as the flight model and systems simulation.
For flight modeling, this thing feels solid. Perhaps a bit too solid, but it will stall (it will not spin in my tests, but I didn’t try everything.) Loops are very satisfying and stable. The rudder feels limited and at full deflection just kind of hits a wall. I’m not sure how accurate that is, but with aileron compensation you have nothing to fear from really stepping on the rudder.
Engine sounds are fairly good inside, not so great outside. They aren’t bad, they’re just too quiet. The sounds are indeed attenuated when closing the canopy. The engines are quite menacing at full RPM, you do get the feeling of being in a WWII interceptor. The aircraft is quite happy to climb and has a service ceiling of over 40,000ft thanks to the oxygen tank (which is operable.)
Some unique elements: The air pressure and oxygen can both be refilled on the ground with a simple click, but it takes some time to refill. The warmup procedure also feels very true to life because it takes some time (5 minutes per checklists) for all of the temperatures to settle, or namely oil temp.
Interior lighting is unique and quite lovely as there is a UV lamp that swivels into place and regular panel (not instrument) lights. Combined they create a truly unique and lovely night flying experience.
Systems wise, there is not as much depth as I would have hoped. The radio which can be powered via the 4 way battery+dyno switch appears to be fully inop and furthermore, pretty much just a 2d image with some bezels. All weapon systems are inop, which didn’t surprise me but it would also be nice to have something modeled there for the non-Marketplace version (if there is to be one.)
There are some but not many interior sounds. Essentials such as opening and closing the canopy, gear and flaps have sounds but many other elements such as switches and levers do not.
Would I recommend this? It’s hard to say. I would recommend it to someone interested in WWII fighters and/or Swiss and French military history. I would recommend it for vintage aviation enthusiasts but not those expecting full systems depth such as authentic vintage radio emulation. I would not recommend it for someone to whom visuals are paramount. I would not recommend it to a super casual flier for that same reason. Overall, if you’re anything like me you’ll likely find something to enjoy in this addon.
Bugs: I only found one bug which is a texture artifact. Near the fuel lever handles there’s a duplicated texture of a good chunk of the panel which kind of funny once you see it but also the type of bug you’d expect a developer to catch and fix before release.
I streamed this aircraft for about an hour and a half here: Morane-Saulnier D.3801 for MSFS - live first impressions - YouTube
Apologies that I’m practically screaming into the mic for the first bit, I had no idea my levels were up that high 
I also had the sound off for the engine start, but caught it so keep watching to hear that. Startup proper is at 35:45 ish.
The texture bug I referred to: 