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Who's tried a non-prototype? Who has bought one? How do you like it? How is the build quality and playing characteristics?

I've got mine one week ago. It arrived in a immaculate looking box (thank you, UPS!) and also undamaged condition.
The silverplating is quite shiny and except one small blemish (which you only see while playing!) it has a great finish. All connections look clean, the tuning slides are moving with a smooth feeling (after lubricating with Selmer tuning slide grease) and fit perfectly. Vented valves move effortlessly but need to wear in a bit to get the smoothness I am used from my older instruments. Before playing I had to remove some excessive silver in the mouthpiece reciever until the mouthpiece would go in.
Playing is great, a big sound and good attack, especially in the low register. It feels quite similar to my 1940 Courtois, but with the big shank mp reciever it is not directly comparable. But this makes live a lot easier with a big choice of mouthpieces. Actually I play it with a Denis Wick Heritage 0AL (Bass Trombone) mouthpiece. There came two mouthpieces with the horn, one Euph-sized, one Tuba-sized. Both sound quite nice, but they both have a very uneven inside cup surface, seems a bit strange. Even with the quite big mouthpiece it plays in tune up to g', but for Bydlo of course the smaller one is the one. Than it stays in tune up to the sky... The difference in sound between the two is really great. Like Arnold Jacobs said, it is easier to carry one tuba and two mouthpieces than the other way round...
Used to a vintage one I play the original fingering with the two-whole-step third and long half-step fifth valve. I just pulled the third slide out, it is long enough. The convertible piece Couesnon offered with the template instrument would be nice!
The hard case looks pretty good as well!

Thank you for the thorough review! It's especially helpful to hear your insights regarding comparisons to period instruments.

Make sure you clean your horn BEFORE you play those valves too much. Some of that "wearing in" is grease and rougue used to buff the horn and it is likely doing a number on your valves. This is not always true, but I have cleaned every new horn I've gotten and never ended up with a break in period.

Of course I spend a lot of time now with cleaning and re-oiling the machine. It is only my first new-bought valved instrument 25 years after my Besson Eb, so it feels a bit different. The Besson felt the same back then... After half a year of intense oiling it needed only some drops every two or three months (or as a routine before a gig).
The Wessex valves look very well machined so I hope they will do their job the same way. Only the valve caps need a bit of attention, sometimes they are reluctant to fit their threads.