I'm traveling in the UK this week and despite my best attempts I'm flying through Heathrow thrice.
I agree with many others - Heathrow pretty much sucks. Too big, too busy, and too disorganized. Even the bits that work well, like the train shuttle to Paddington in central London, doesn't really make traveling easier. Paddington connects with the London Underground, where you quickly grind to a dimly-lit halt somewhere short of Kings Cross.
I'm so glad I don't commute in London anymore.