As there are not a lot of reviews of this very good product, here goes mine of a trip done in March.
The flight was part of a trip ORY-EWR//YUL-LHR-ORY with my brother. This was a vacation where we drove from New York to Montreal via Providence, Boston, Salem and Portland. There will be a full TR.
My brother being of the type that flying is just a mean to an end and who does not care for comfort but rather efficiency, there was no ex-DUB or ex-EU and the whole trip was in WT/ET, booked on IB website as they asked 200 EUR less than BA for the same itinerary on the same dates.
It did not create any issues except that the BA App was close to useless.

At ORY I enquired for an AUP from Eco to Prem Eco ( OpenSkies lingo for WT and WTP). After parting with two times 150 EUR, we had boarding passes for seats in Prem.

Checkin and security: this was quick and efficient. The line for Biz bed/BAEC elites was empty. The staff at ORY handling BA is always service minded and it showed again.

Lounge: this is my third or fourth passage in this lounge since its opening less than a year ago and I like it. There was enough seating, the snacks are good and the staff are pro active filling the buffet or passing around asking if you need anything. The Wifi was way better compared to my previous visits.

Boarding: priority boarding started without announcement and a semi-enforcement. An effort was made without heart. Nevertheless this was quickly done and we were soon ready to leave.

Plane and seating: OpenSkies operates 757, divided into Biz Bed (old CW), Prem Eco in 2*2 confit and Eco in 3*3. The Prem Eco seats are very similar (if not exactly the same ) to AA 737 First seats. They have also a leg extension with Foot rest, and reclines a lot (45 degrees would be my guess). There are plenty of leg room and this is very adequate for a day time TATL to the East Coast. The arm rests are large, the seat pockets deep. There are no curtains between Prem Eco and Eco so this does not have the small cabin feeling that WTP does.
My main complaints would be that there are only 2 toilets for the two sections and at times the queue is rather long. Also the plane is and feel old. Not a problem in any way but do not expect a shiny new cabin.

Food/service: a bottle of water was waiting in our seat, and there was passage by the FA frequently during the flight for water or juice. We had a first bar service just after take off, then the meal. The meal is the same for Eco and Prem. I would guess Biz Bed gets a different service. However this is not a standard BA WT fare. First metal cutlery is provided. All containers, cups and glasses are China, not plastic. A cloth napkin completes the set. Food wise this was good, I had pasta and mushroom for my main and this was at the level of a CE band 4 meal in term of quality and quantity. Coffee completed the service until the snack prior to landing, which was also good compared to the usual.

IFE: Open Skies provided pre loaded iPads. The selection is poor compared to the normal BA IFE. Be prepared to bring your own. I watched Star Wars VII and read a book, the time passed quickly enough.

Conclusion: a very good product. The seating provides an experience similar to a BA seat, however the service and soft product is a notch above. I would definitely recommend it for a day flight considering that fares can be pretty low, and the 149 EUR for an AUP to Prem is the standard price. The connection needed at ORY if coming from LHR is fairly simple.