From the scotsman. I'd somehow missed this! I go back and forth to London at least every three months, sometimes more, and after trying both for the first few years, have definitely moved to train as my default option - partly because my destination is usually walk-able from King's Cross. My only complaint is that my most regular travel day is Friday, and the trains are definitely more crowded both morning and evening, and the late return trains can be pretty grim.
Will be interesting to see if anyone in these debates mentions being able to take a bike with you as a benefit of train over plane.