Spent 4 days in London - and used the Underground many, many times... fast, efficient, clean, and very, very convenient to get to any nook and corner of the great city. With hundreds of miles of tunnels criss crossing each other, you are never a few meters away from a tube station, and this is by far the best public transport I have seen in my life.
Also amazing is the signages - everything is crystal clear, you never have to ask! Just look around, wherever you are, and you know exactly what you want to do next! The trains tell you where you are, what's coming up. The stations clearly point to the exits and the change points for the other lines. And very, very helpful transit officers will assist you with the ticketing, and holding the gates open for you if you are unsure how to use the ticket stubs to do so.
And unlike in most parts of the US, it's extremely safe, even at midnight - none of the scary, lonely stations with crazed and drugged teens hanging around...
Pretty darn impressive!