It was fine for me. Got the 6:15 tube from Brixton to Oxford Circus, changed for the central line to get to Stratford, DLR from there to Excel. I actually ended up arriving half an hour earlier than I planned.
You must get a leaflet from the station and go on the 2 for 1 offer. so as long as ther are 2 of you its half price. Def not worth the full price at that age.