“If the phone is in normal use without playing games or calling for long time, the temperature should be around 37°C or 38°C. But if you use your phone to play games or surfing internet for a long time, it will be normal that the temperature rises to around 55°C.“, official statement from HTC.
I was worried after discovered my HTC One X is overheating up to 53.5 °C. Consulted HTC Support. Was asked to do a battery test. And found out that my HTC One X battery is performing well. So I double confirm with HTC Support again. And got the statement as above.