Was in two minds about booking with Primera due to the generally negative feedback. Went ahead anyway as the price was so good (£290 per return flight Birmingham to New York).
Overall had a good experience - check in Birmingham was great and I believe as we had my elderly mother us we were assigned seats near to the main exit on the plane which was great for boarding and getting off. Flights were more or less on time both ways (although security at Newark took forever...) Check in at Newark on the return not quite such good service with a bit of an aggressive undertone from the woman at the check in desk that seems prevalent at that airport. (Apparently staff at the check in desks are not actually Primera employees but are contracted by the airline.)
Plane was clean and was happy for it to be no frills (e.g.didn't take the meal option (extra cost) but took on some nice food from Waitrose before the flight), to and pay for the odd cup of tea, for the good ticket price. Service was fine.
I also contacted Primera by email about taking an instrument on the plane and got a polite, friendly and timely response to my queries. Ended up posting it as it was a fair bit cheaper, but the option was there online in my booking to add it as 'sports equipment'. Maybe Primera should add a musical instrument option to avoid confusion and people having to ask about instruments.
Overall a good experience and a very competitive ticket price.