We went one night when Justin Bieber was playing the O2 (we did not realise it was that night). The queue was out of the door, but the organisation in there is great and it wasn't long at all before we had our food.
The portion sizes are huge and we could have easily shared a large fries between us. Really nice milkshakes as well. The food is good. It is fast food, so not exactly gourmet, but it is a really tasty burger and cooked fresh behind the counter so its hot as well.