Used STN a fair bit and have had no problem with check-in queues or feeling that the terminal was any dirtier than LHR. In fact my vote for the most grubby terminal has to go to LGW's South Terminal - dark, dank with the EZY check-in an absolute nightmare on occasion. Must confess though I have not flown late night or early morning so can't comment on the 'overnighters' at STN.