I was there on a Saturday morning and it was a zoo. Lots of holidaymakers and not enough seats - a situation all the worse because of maintenance work going on. Forget about sitting down next to a socket either. Very poor for recharging.
Couple of bright spots - the Dixons set up on the first floor (using shuttered units rather than a proper shop) is actually v usable and cool - there is aircon right overhead. Also on the first floor is the BT Openzone sit down area.
Still disappointing not to find any free wi-fi around. But all in all, a worse wi-fi travel experience than Heathrow.