I have flown with them three times. Very nice experience on all three occasions. Text message to confirm you are checked in, text message to let you know if they running behind schedule, well turned out crew, Free WiFi on board, and three less seats for extra leg room. And no "big sell" via the dreaded PA. Aircraft was full on all three occasions with 180 or so very content passengers.
They are the third biggest budget operator in Europe and are streets ahead of Easy and worlds away from Ryanair with regard to passenger experience IMHO.
Weather it's sustainable remains to be seen.