Has anyone flown PE with BMI from MAN-ORD?
How does it compare to PE with Virgin & BA's WT+?

Does anyone know if they'll still be using their Airbus next June or is it being downgraded to a 767?

Not sure about the aircraft question sorry, although I doubt that they would downgrade to a smaller plane as I think that it is a really popular long haul route for BMI and is a code share with LH and UA.
I've flown PE a few times with BMI, and have flown WT+ once with BA.
With BMI the seats are the same (whereas WT+ and Virgin both have wider seats) but with the extra 6" of legroom which makes a massive difference if you are a taller traveller.
I've always got good service with BMI (much better than when I flew WT+), the crew seem very attentive as it is only a relatively small cabin and you get a couple of extra features on normal economy such as priority boarding and checkin, juice/water when you board and a power point for laptops.
I don't think that it deserves a massive premium on the normal economy ticket price, but if you can get it for not much more (or free if you are BD * Gold) then it is definitely worth it.

You are likely to get more answers in the BMI forum. BD's PE is not really comparable to BA's WT+ or VS's PE. The extra room and smaller cabin is just about all you get extra to economy (oh, and free booze when BD start to charge for alcoholic drinks in ordinary economy - this January, I think).
Not really worth paying more than ordinary economy, unless the difference in fare happens to be minimal.