Anyone that thinks 240hz is useful is a nutcase. Show me a double-blind test that proves it. People will go far defending their outrageous purchases however.
On CRT monitors the difference between 120 and 200Hz was very noticeable in fast arena shooters. On LCDs with their inherent motion blur there's probably less of a difference between 144 and 240Hz, but I'm not going to call it placebo due to my previous experience with high frequency CRTs.