Yes, I did once (2013). It was a mattress from an IKEA sofa bed. It was a foam one, so it just about fit into the back of the car with the seats down when rolled in half. We were stating in Worthy View so getting it from the car park to the tent was doable (no way I could have lugged onsite form the normal car parks). We bought king sized duvet and pillows too. It fit quite snugly into the pre-erected tent at Worthy View. Also where we were sited was behind a hill which blocked out the sound of the fest. So when in the tent it felt like sleeping in a proper bed. I have never slept so well at a festival.
There were times late at night when I feeling a bit weary when I started to think, actually I quite fancy going back to the tent for a cosy sleep. That's not a feeling I have ever had at a festival before or since!