Once you've done the tunnels you'll never do the train again. I like the solitude, especially compared to the hectic mayhem of the 5A. Then there's the benefit of the leg stretch, and of course the warm glow that you somehow have special knowledge of the airport. One other benefit for the petty like myself, is as someone else once mentioned, you press the lift button and no one else checks, the lift then arrives downstairs and there's this short moment of confusion as the herd leaves the lift and realises it's not what they expected.